add docs example for Ingest scripts manipulating document metadata (#24875)

It may not be clear to users that the Ingest ScriptProcessor context object `ctx` can 
manipulate document metadata like `_index` and `_type`.

docs/reference/ingest/ingest-node.asciidoc

@@ -1738,6 +1738,63 @@ numeric fields `field_a` and `field_b` multiplied by the parameter param_c:
 --------------------------------------------------
 // NOTCONSOLE
 
+It is possible to use the Script Processor to manipulate document metadata like `_index` and `_type` during
+ingestion. Here is an example of an Ingest Pipeline that renames the index and type to `my_index` no matter what
+was provided in the original index request:
+
+[source,js]
+--------------------------------------------------
+PUT _ingest/pipeline/my_index
+{
+    "description": "use index:my_index and type:my_type",
+    "processors": [
+      {
+        "script": {
+          "inline": " ctx._index = 'my_index'; ctx._type = 'my_type' "
+        }
+      }
+    ]
+}
+--------------------------------------------------
+// CONSOLE
+
+Using the above pipeline, we can attempt to index a document into the `any_index` index.
+
+[source,js]
+--------------------------------------------------
+PUT any_index/any_type/1?pipeline=my_index
+{
+  "message": "text"
+}
+--------------------------------------------------
+// CONSOLE
+// TEST[continued]
+
+The response from the above index request:
+
+[source,js]
+--------------------------------------------------
+{
+  "_index": "my_index",
+  "_type": "my_type",
+  "_id": "1",
+  "_version": 1,
+  "result": "created",
+  "_shards": {
+    "total": 2,
+    "successful": 1,
+    "failed": 0
+  },
+  "_seq_no": 0,
+  "_primary_term": 1,
+  "created": true
+}
+--------------------------------------------------
+// TESTRESPONSE
+
+In the above response, you can see that our document was actually indexed into `my_index` instead of
+`any_index`. This type of manipulation is often convenient in pipelines that have various branches of transformation,
+and depending on the progress made, indexed into different indices.
 
 [[set-processor]]
 === Set Processor