[DOCS] Defined es-test-dir and plugins-examples-dir in index.asciidoc. (#25232)

Use these attributes when specifying the location of included tests.

docs/reference/analysis/tokenfilters/stemmer-override-tokenfilter.asciidoc

@@ -46,7 +46,7 @@ Where the file looks like:
 
 [source,stemmer_override]
 --------------------------------------------------
-include::{docdir}/../src/test/cluster/config/analysis/stemmer_override.txt[]
+include::{es-test-dir}/cluster/config/analysis/stemmer_override.txt[]
 --------------------------------------------------
 
 You can also define the overrides rules inline:

docs/reference/analysis/tokenfilters/synonym-graph-tokenfilter.asciidoc

@@ -65,7 +65,7 @@ The following is a sample format of the file:
 
 [source,synonyms]
 --------------------------------------------------
-include::{docdir}/../src/test/cluster/config/analysis/synonym.txt[]
+include::{es-test-dir}/cluster/config/analysis/synonym.txt[]
 --------------------------------------------------
 
 You can also define synonyms for the filter directly in the

docs/reference/analysis/tokenfilters/synonym-tokenfilter.asciidoc

@@ -49,7 +49,7 @@ The following is a sample format of the file:
 
 [source,synonyms]
 --------------------------------------------------
-include::{docdir}/../src/test/cluster/config/analysis/synonym.txt[]
+include::{es-test-dir}/cluster/config/analysis/synonym.txt[]
 --------------------------------------------------
 
 You can also define synonyms for the filter directly in the

docs/reference/index.asciidoc

@@ -1,4 +1,7 @@
 [[elasticsearch-reference]]
 = Elasticsearch Reference
 
+:es-test-dir: {docdir}/../src/test
+:plugins-examples-dir: {docdir}/../../plugins/examples
+
 include::index-shared.asciidoc[]

docs/reference/modules/scripting/engine.asciidoc

@@ -17,7 +17,7 @@ the document frequency of a provided term.
 
 ["source","java",subs="attributes,callouts,macros"]
 --------------------------------------------------
-include-tagged::{docdir}/../../plugins/examples/script-expert-scoring/src/main/java/org/elasticsearch/example/expertscript/ExpertScriptPlugin.java[expert_engine]
+include-tagged::{plugins-examples-dir}/script-expert-scoring/src/main/java/org/elasticsearch/example/expertscript/ExpertScriptPlugin.java[expert_engine]
 --------------------------------------------------
 
 You can execute the script by specifying its `lang` as `expert_scripts`, and the name