[DOCS] Add examples to API endpoints

[szabosteve]

Description

Add examples to the API endpoints.

• Use YAML format instead of JSON, since it supports comments. For example, conversion can be done via https://marketplace.visualstudio.com/items?itemName=tuxtina.json2yaml
• Put the examples in "request" and "response" subfolders alongside the appropriate specifications. If you know the response code, use a "200_response" subfolder, for example.
• 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 Add bulk, count, clear scroll, close PIT examples #3510
  • clear scroll Add bulk, count, clear scroll, close PIT examples #3510
  • close_point_in_time Add bulk, count, clear scroll, close PIT examples #3510
  • open_point_in_time Add examples to search and open PIT APIs #3561
  • count Add bulk, count, clear scroll, close PIT examples #3510
  • create [DOCS] Add examples for create, delete, index, reindex APIs #3540
  • delete [DOCS] Add examples for create, delete, index, reindex APIs #3540
  • delete_by_query/delete_by_query_rethrottle [DOCS] Add examples for delete by query API #3574
  • update_by_query/update_by_query_rethrottle [DOCS] Add examples for update by query #3597
  • delete_script/get_script/put_script [DOCS] Add examples and descriptions for script APIs #3613
  • scripts_painless_execute Augment painless execute API #3589
  • exists/exists_source [DOCS] Add examples for create, delete, index, reindex APIs #3540
  • explain [DOCS] Add examples and descriptions for explain, termvector, search APIs #3614
  • index [DOCS] Add examples for create, delete, index, reindex APIs #3540
  • msearch_template [DOCS] Add examples for mget, msearch template APIs #3615
  • mtermvectors/termvectors [DOCS] Add examples and descriptions for explain, termvector, search APIs #3614
  • query rules Add query rule API examples #3439
  • reindex [DOCS] Add examples for create, delete, index, reindex APIs #3540
  • reindex_rethrottle [DOCS] Add examples for create, delete, index, reindex APIs #3540
  • render_search_template [DOCS] Add examples and descriptions for explain, termvector, search APIs #3614
  • search_shards [DOCS] Add examples and descriptions for explain, termvector, search APIs #3614
  • search_template [DOCS] Add examples and descriptions for explain, termvector, search APIs #3614
  • search Add examples to search and open PIT APIs #3561
  • queries N/A
  • aggs
  • suggesters N/A
• async_search Adds async search API request and response examples #2780
• autoscaling Adds autoscaling and CCR API request and response examples #2788
• cat Add cat response examples #3419
• ccr Adds autoscaling and CCR API request and response examples #2788
• cluster Adds cluster API example requests and responses #2870
• component_templates
• connector Add connector API examples #3428
• data stream Add datastream API examples #3429
• 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 Add more index API examples #3430
• ingest Adds ingest API request and response examples #2996
• license Add examples to licensing APIs #3420
• logstash Add Logstash API examples #3436
• migration Add migration API examples #3437
• 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 - Only contains API with visibility=private
• nodes Add shutdown API examples #3438, Add reload search analyzer, root and xpack API examples #3454
• rollup Add rollup API examples #3455
• search_application Add specification for post event to an analytics collection #3363, [DOCS] Add behavioral analytics and search application examples #3392, Add search_application.render_query #3401
• searchable_snapshots Add searchable snapshot API examples #3459
• security Add security API examples #3490, [DOCS] Add examples for security user profile and SAML APIs #3520, [DOCS] Add remaining security API examples #3546
• shutdown Add shutdown API examples #3438
• slm Add SLM API examples #3470
• snapshot Add snapshot and restore examples #3463
• sql Add SQL API examples #3479
• ssl Add security API examples #3490
• synonyms Add synonym API examples #3486
• tasks Add task examples #3509
• text_structure Add text structure API examples #3487
• transform Add examples for transform APIs #2505
• watcher Add Watcher API examples #3489
• xpack Add reload search analyzer, root and xpack API examples #3454 Add info and usage API examples #3488

[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