[DOCS][ESQL] Cleanup and cross-reference LOOKUP JOIN reference and landing pages (#127215)

* [DOCS][ESQL] Cleanup and cross-reference LOOKUP JOIN reference and landing pages

**lookup-join.md (syntax reference)**:
- removed tip formatting for simpler direct link to landing page
- improved parameter formatting and descriptions
- fixed template variable from `{esql}` to `{{esql}}`

**esql-lookup-join.md (landing page)**:
- added "compare with enrich" section header
- simplified "how the command works" with clearer parameter explanation
- added code example in how it works section
- improved image alt text for accessibility
- organized example section with better context and SQL comparison
- added dropdown for sample tables to reduce visual clutter
- added "query" subheading for clearer organization
- included reference to additional examples in command reference
- removed excessive whitespace

* Improve example, add setup code

replaced abstract employee/language example with security monitoring use case
added setup instructions for creating test indices
included sample data loading via bulk api
new practical query example joining firewall logs with threat data
simplified results output showing threat detection scenario
added note about left-join behavior
improved code comments and structure
added required index.mode: lookup setting info

docs/reference/query-languages/esql/_snippets/commands/layout/lookup-join.md

@@ -11,6 +11,8 @@ SLA of official GA features.
 index, to your {{esql}} query results, simplifying data enrichment
 and analysis workflows.
 
+Refer to [the high-level landing page](../../../../esql/esql-lookup-join.md) for an overview of the `LOOKUP JOIN` command, including use cases, prerequisites, and current limitations.
+
 **Syntax**
 
 ```esql
@@ -21,18 +23,14 @@ FROM <source_index>
 **Parameters**
 
 `<lookup_index>`
-: The name of the lookup index. This must be a specific index name - wildcards, aliases, and remote cluster
-  references are not supported.
+:   The name of the lookup index. This must be a specific index name - wildcards, aliases, and remote cluster references are not supported. Indices used for lookups must be configured with the [`lookup` index mode](/reference/elasticsearch/index-settings/index-modules.md#index-mode-setting).
 
 `<field_name>`
-: The field to join on. This field must exist
-  in both your current query results and in the lookup index. If the field
-  contains multi-valued entries, those entries will not match anything
-  (the added fields will contain `null` for those rows).
+:   The field to join on. This field must exist in both your current query results and in the lookup index. If the field contains multi-valued entries, those entries will not match anything (the added fields will contain `null` for those rows).
 
 **Description**
 
-The `LOOKUP JOIN` command adds new columns to your {esql} query
+The `LOOKUP JOIN` command adds new columns to your {{esql}} query
 results table by finding documents in a lookup index that share the same
 join field value as your result rows.
 

docs/reference/query-languages/esql/esql-lookup-join.md

@@ -16,7 +16,9 @@ For example, you can use `LOOKUP JOIN` to:
 * Quickly see if any source IPs match known malicious addresses.
 * Tag logs with the owning team or escalation info for faster triage and incident response.
 
-[`LOOKUP join`](/reference/query-languages/esql/commands/processing-commands.md#esql-lookup-join) is similar to [`ENRICH`](/reference/query-languages/esql/commands/processing-commands.md#esql-enrich) in the fact that they both help you join data together. You should use `LOOKUP JOIN` when:
+## Compare with `ENRICH`
+
+[`LOOKUP JOIN`](/reference/query-languages/esql/commands/processing-commands.md#esql-lookup-join) is similar to [`ENRICH`](/reference/query-languages/esql/commands/processing-commands.md#esql-enrich) in the fact that they both help you join data together. You should use `LOOKUP JOIN` when:
 
 * Your enrichment data changes frequently
 * You want to avoid index-time processing
@@ -26,82 +28,119 @@ For example, you can use `LOOKUP JOIN` to:
 * You want to restrict users to use only specific lookup indices
 * You do not need to match using ranges or spatial relations
 
-## How the `LOOKUP JOIN` command works [esql-how-lookup-join-works]
+## How the command works [esql-how-lookup-join-works]
+
+The `LOOKUP JOIN` command adds fields from the lookup index as new columns to your results table based on matching values in the join field.
+
+The command requires two parameters:
+- The name of the lookup index (which must have the `lookup` [`index.mode setting`](/reference/elasticsearch/index-settings/index-modules.md#index-mode-setting))
+- The name of the field to join on
 
-The `LOOKUP JOIN` command adds new columns to a table, with data from {{es}} indices.
+```esql
+LOOKUP JOIN <lookup_index> ON <field_name>
+```
 
 :::{image} ../images/esql-lookup-join.png
-:alt: esql lookup join
+:alt: Illustration of the `LOOKUP JOIN` command, where the input table is joined with a lookup index to create an enriched output table.
 :::
 
-`<lookup_index>`
-: The name of the lookup index. This must be a specific index name - wildcards, aliases, and remote cluster references are not supported. Indices used for lookups must be configured with the [`lookup` index mode](/reference/elasticsearch/index-settings/index-modules.md#index-mode-setting).
-
-`<field_name>`
-: The field to join on. This field must exist in both your current query results and in the lookup index. If the field contains multi-valued entries, those entries will not match anything (the added fields will contain `null` for those rows).
+If you're familiar with SQL, `LOOKUP JOIN` has left-join behavior. This means that if no rows match in the lookup index, the incoming row is retained and `null`s are added. If many rows in the lookup index match, `LOOKUP JOIN` adds one row per match.
 
 ## Example
 
-`LOOKUP JOIN` has left-join behavior. If no rows match in the lookup index, `LOOKUP JOIN` retains the incoming row and adds `null`s. If many rows in the lookup index match, `LOOKUP JOIN` adds one row per match.
-
-In this example, we have two sample tables:
+You can run this example for yourself if you'd like to see how it works, by setting up the indices and adding sample data.
+
+### Sample data
+:::{dropdown} Expand for setup instructions
+
+**Set up indices**
+
+First let's create two indices with mappings: `threat_list` and `firewall_logs`.
+
+```console
+PUT threat_list
+{
+  "settings": {
+    "index.mode": "lookup" # The lookup index must use this mode
+  },
+  "mappings": {
+    "properties": {
+      "source.ip": { "type": "ip" },
+      "threat_level": { "type": "keyword" },
+      "threat_type": { "type": "keyword" },
+      "last_updated": { "type": "date" }
+    }
+  }
+}
+```
+```console
+PUT firewall_logs
+{
+  "mappings": {
+    "properties": {
+      "timestamp": { "type": "date" },
+      "source.ip": { "type": "ip" },
+      "destination.ip": { "type": "ip" },
+      "action": { "type": "keyword" },
+      "bytes_transferred": { "type": "long" }
+    }
+  }
+}
+```
 
-**employees**
+**Add sample data**
 
-| birth_date|emp_no|first_name|gender|hire_date|language|
-|---|---|---|---|---|---|
-|1955-10-04T00:00:00Z|10091|Amabile    |M|1992-11-18T00:00:00Z|3|
-|1964-10-18T00:00:00Z|10092|Valdiodio  |F|1989-09-22T00:00:00Z|1|
-|1964-06-11T00:00:00Z|10093|Sailaja    |M|1996-11-05T00:00:00Z|3|
-|1957-05-25T00:00:00Z|10094|Arumugam   |F|1987-04-18T00:00:00Z|5|
-|1965-01-03T00:00:00Z|10095|Hilari     |M|1986-07-15T00:00:00Z|4|
+Next, let's add some sample data to both indices. The `threat_list` index contains known malicious IPs, while the `firewall_logs` index contains logs of network traffic.
 
-**languages_non_unique_key**
+```console
+POST threat_list/_bulk
+{"index":{}}
+{"source.ip":"203.0.113.5","threat_level":"high","threat_type":"C2_SERVER","last_updated":"2025-04-22"}
+{"index":{}}
+{"source.ip":"198.51.100.2","threat_level":"medium","threat_type":"SCANNER","last_updated":"2025-04-23"}
+```
 
-|language_code|language_name|country|
-|---|---|---|
-|1|English|Canada|
-|1|English|
-|1||United Kingdom|
-|1|English|United States of America|
-|2|German|[Germany\|Austria]|
-|2|German|Switzerland|
-|2|German|
-|4|Spanish|
-|5||France|
-|[6\|7]|Mv-Lang|Mv-Land|
-|[7\|8]|Mv-Lang2|Mv-Land2|
-||Null-Lang|Null-Land|
-||Null-Lang2|Null-Land2|
+```console
+POST firewall_logs/_bulk
+{"index":{}}
+{"timestamp":"2025-04-23T10:00:01Z","source.ip":"192.0.2.1","destination.ip":"10.0.0.100","action":"allow","bytes_transferred":1024}
+{"index":{}}
+{"timestamp":"2025-04-23T10:00:05Z","source.ip":"203.0.113.5","destination.ip":"10.0.0.55","action":"allow","bytes_transferred":2048}
+{"index":{}}
+{"timestamp":"2025-04-23T10:00:08Z","source.ip":"198.51.100.2","destination.ip":"10.0.0.200","action":"block","bytes_transferred":0}
+{"index":{}}
+{"timestamp":"2025-04-23T10:00:15Z","source.ip":"203.0.113.5","destination.ip":"10.0.0.44","action":"allow","bytes_transferred":4096}
+{"index":{}}
+{"timestamp":"2025-04-23T10:00:30Z","source.ip":"192.0.2.1","destination.ip":"10.0.0.100","action":"allow","bytes_transferred":512}
+```
+:::
 
-Running the following query would provide the results shown below.
+### Query the data
 
 ```esql
-FROM employees
-| EVAL language_code = emp_no % 10
-| LOOKUP JOIN languages_lookup_non_unique_key ON language_code
-| WHERE emp_no > 10090 AND emp_no < 10096
-| SORT emp_no, country
-| KEEP emp_no, language_code, language_name, country;
+FROM firewall_logs # The source index
+| LOOKUP JOIN threat_list ON source.ip # The lookup index and join field
+| WHERE threat_level IS NOT NULL # Filter for rows non-null threat levels
+| SORT timestamp # LOOKUP JOIN does not guarantee output order, so you must explicitly sort the results if needed
+| KEEP timestamp, source.ip, destination.ip, action, threat_level, threat_type # Keep only relevant fields
+| LIMIT 10 # Limit the output to 10 rows
 ```
 
-|emp_no|language_code|language_name|country|
-|---|---|---|---|
-|    10091      | 1                     | English               | Canada|
-|    10091      | 1                     | null                  | United Kingdom|
-|    10091      | 1                     | English               | United States of America|
-|    10091      | 1                     | English               | null|
-|    10092      | 2                     | German                | [Germany, Austria]|
-|    10092      | 2                     | German                | Switzerland|
-|    10092      | 2                     | German                | null|
-|    10093      | 3                     | null                  | null|
-|    10094      | 4                     | Spanish               | null|
-|    10095      | 5                     | null                  | France|
-
-::::{important}
-`LOOKUP JOIN` does not guarantee the output to be in any particular order. If a certain order is required, users should use a [`SORT`](/reference/query-languages/esql/commands/processing-commands.md#esql-sort) somewhere after the `LOOKUP JOIN`.
-
-::::
+### Response
+
+A successful query will output a table. In this example, you can see that the `source.ip` field from the `firewall_logs` index is matched with the `source.ip` field in the `threat_list` index, and the corresponding `threat_level` and `threat_type` fields are added to the output.
+
+```
+   source.ip   |    action     |  threat_type  | threat_level  
+---------------+---------------+---------------+---------------
+203.0.113.5    |allow          |C2_SERVER      |high           
+198.51.100.2   |block          |SCANNER        |medium         
+203.0.113.5    |allow          |C2_SERVER      |high        
+```
+
+### Additional examples
+
+Refer to the examples section of the [`LOOKUP JOIN`](/reference/query-languages/esql/commands/processing-commands.md#esql-lookup-join) command reference for more examples.
 
 ## Prerequisites [esql-lookup-join-prereqs]