[DOCS] Add examples to API endpoints

[szabosteve]

Description

Add examples to the API endpoints.

• Use JSON format. Example PR: Add machine learning API examples #2452
• To prevent duplicated work, leave your name beside a namespace/API you start working on.
• The list contains only the API namespaces (for example, indices), you can expand the list granularly by adding the endpoints related to the namespace you work on in case it helps track the effort (for example, if you don't cover the whole namespace in one PR).
• Add your PR URL next to the related namespace/API.
• Adjust the list if you find any missing namespaces/endpoints.
• Leave relevant notes either in the list or in a comment.

NOTE: Until there's an automated method for associating examples with endpoints, we're accomplishing it via overlays. Reach out to @lcawl if assistance is required.

API namespace inventory:

• _global
  • bulk
  • close_point_in_time/open_point_in_time
  • create
  • delete
  • delete_by_query/delete_by_query_rethrottle/update_by_query/update_by_query_rethrottle
  • delete_script/get_script/put_script/scripts_painless_execute
  • exists/exists_source
  • explain
  • index
  • msearch_template
  • mtermvectors/termvectors
  • reindex_rethrottle
  • render_search_template
  • search_shards
  • search_template
  • search
  • queries
  • aggs
  • suggesters
• async_search Adds async search API request and response examples #2780
• autoscaling Adds autoscaling and CCR API request and response examples #2788
• cat
• ccr Adds autoscaling and CCR API request and response examples #2788
• cluster Adds cluster API example requests and responses #2870
• component_templates
• count
• data stream
• enrich
• eql Adds EQL, ESQL, features API example requests and responses #2879
• es|ql Adds EQL, ESQL, features API example requests and responses #2879
• features Adds EQL, ESQL, features API example requests and responses #2879
• fleet
• graph Adds graph and ILM API example requests and responses #2900
• ilm Adds graph and ILM API example requests and responses #2900
• indices Adds index API requests and responses - part 1 #2979 Adds index API requests and responses - part 2 #2993
• ingest Adds ingest API request and response examples #2996
• license
• logstash
• migration
• ml Add machine learning API examples #2452, [DOCS] Adds ML API examples (DELETE ops) #2518, [DOCS] Adds example requests and responses to ML APIs (C-E) #2521, ...
• monitoring
• nodes
• reindex
• rollup
• search_application
• searchable_snapshots
• security
• shutdown
• slm
• snapshot
• sql
• ssl
• tasks
• text_structure
• transform Add examples for transform APIs #2505
• watcher
• xpack

[lcawl]

In case it's useful to other folks wanting a small (temporary) preview document, https://docs.bump.sh/guides/openapi/augmenting-generated-openapi/ mentions https://github.com/Mermade/openapi-filter/, and I was able to use it to strip out smaller subsets of the document. For example, to pull out only the operations that have the tag "transform.put_transform"):

npm install --global openapi-filter
make overlay-docs
openapi-filter -i --flags transform.put_transform --checkTags --info --valid -- output/openapi/elasticsearch-serverless-openapi.new.json output/openapi/filtered.json
bump preview output/openapi/filtered.json

[szabosteve]

Related PR that simplifies the folder structure: #2779