Split ES|QL functions/operators/commands into separate pages for similar functions and make commands examples generated

[craigtaverner]

Fixes #126199

While the internal structure of the docs is already split into many (over 1000) sub-pages, the final display for the Functions and Operators page is a single giant page, making navigation harder. This PR splits it into separate pages, one for each group of similar functions and one for the operators. Twelve new pages.

This PR also bundles a few other related changes. In total what is done is:

• Split functions/operators into 12 pages, one for each group, maintaining the existing split of each function/operator into a snippet with dynamically generated examples
• Split esql-commands.md into source-commands.md and processing-commands.md, each of which is split into individual snippets, one for each command
• Each command snippet has it's examples split out into separate files, if they were examples that were dynamically generated in the older asciidoc system
• The examples files are overwritten by the ES|QL unit tests, using a similar mechanism to the examples written for functions and operators)
• Some additional refinements to the Kibana definition and markdown files (nicer operator headings, and display text)

For example, this is what it looks like to navigate to MV_MEDIAN, where we can see on the left the new tree-structure with a separate item for multivalue functions and a shorter contents list on the right showing all the mv-functions:

The same is done for the commands page, in preparation for also making all examples in commands based on tested examples again.

[elasticsearchmachine]

Pinging @elastic/es-docs (Team:Docs)

[elasticsearchmachine]

Pinging @elastic/es-analytical-engine (Team:Analytics)

[alex-spies]

I didn't check the details, but I agree with the approach and new structure. Thanks @craigtaverner !

[leemthompo]

Few suggestions to capitalize nav titles and fix page mappings

[leemthompo]

Suggested change

	- https://www.elastic.co/guide/en/elasticsearch/reference/current/functions-operators/aggregation-functions.html
	- https://www.elastic.co/guide/en/elasticsearch/reference/current/esql-functions-operators.html#esql-agg-functions

This metadata maps pages from old system to new system, and they have been impacted by find/replaces elsewhere

[craigtaverner]

So the url to use for mapped_pages should be the old URL? I can make that change. I had thought I would need to add a bunch of entries to redirects, but perhaps this has the same effect?

[leemthompo]

@craigtaverner this is purely for migration purposes and is unrelated to redirects.yml

[leemthompo]

Otherwise, this is looking great @craigtaverner 👏. I think we'll just need to set up a temporary redirects.yml file to mute the errors of the following type, due to URL path changes:

'reference/query-languages/esql/esql-commands.md' has no anchor named: '#esql-stats-by'. (docs-content:0)

Writers will fix the links in docs-content once this gets merged.

[leemthompo]

I think the redirects.yml file should look something like this:

redirects:
  # Redirect main files that were split up
  'query-languages/esql/esql-commands.md':
    many:
      - to: 'query-languages/esql/commands/source-commands.md'
        anchors:
          'esql-from': 'esql-from'
          'esql-row': 'esql-row'
          'esql-show': 'esql-show'
      - to: 'query-languages/esql/commands/processing-commands.md'
        anchors:
          'esql-stats-by': 'esql-stats-by'
          'esql-limit': 'esql-limit'
          'esql-drop': 'esql-drop'
          'esql-eval': 'esql-eval'
          'esql-keep': 'esql-keep'
          'esql-lookup-join': 'esql-lookup-join'
          'esql-enrich': 'esql-enrich'
          'esql-where': 'esql-where'
          'esql-sort': 'esql-sort'
          'esql-grok': 'esql-grok'
          'esql-dissect': 'esql-dissect'

  # Redirect function and operator anchors
  'query-languages/esql/esql-functions-operators.md':
    many:
      - to: 'query-languages/esql/functions-operators/aggregation-functions.md'
        anchors:
          'esql-agg-functions': ''
      - to: 'query-languages/esql/functions-operators/type-conversion-functions.md'
        anchors:
          'esql-type-conversion-functions': ''
          'esql-to_string': 'esql-to_string'
      - to: 'query-languages/esql/functions-operators/search-functions.md'
        anchors:
          'esql-search-functions': ''
          'esql-match': 'esql-match'
          'esql-kql': 'esql-kql'
          'esql-qstr': 'esql-qstr'
      - to: 'query-languages/esql/functions-operators/operators.md'
        anchors:
          'esql-operators': ''
          'esql-search-operators': 'esql-match-operator'
          'esql-like': 'esql-like'
      - to: 'query-languages/esql/functions-operators/spatial-functions.md'
        anchors:
          'esql-spatial-functions': ''
          'esql-st_distance': 'esql-st_distance'
          'esql-st_within': 'esql-st_within'
          'esql-st_contains': 'esql-st_contains'
          'esql-st_disjoint': 'esql-st_disjoint'
          'esql-st_intersects': 'esql-st_intersects'
      - to: 'query-languages/esql/functions-operators/date-time-functions.md'
        anchors:
          'esql-now': 'esql-now'
      - to: 'query-languages/esql/functions-operators/grouping-functions.md'
        anchors:
          'esql-bucket': 'esql-bucket'
      - to: 'query-languages/esql/functions-operators/math-functions.md'
        anchors:
          'esql-round': 'esql-round'

[leemthompo]

LGTM 👏

[elasticsearchmachine]

💔 Backport failed

Status	Branch	Result
❌	9.0	Commit could not be cherrypicked due to conflicts

You can use sqren/backport to manually backport by running backport --upstream elastic/elasticsearch --pr 126279