ESQL: meta issue: migrate docs to markdown

[alex-spies]

Future docs are going to be markdown-based, making our asciidoc-based docs obsolete. They will be following the syntax described here.

TODO:

• Migrate ESQL's automatic doc generation to respect the new syntax
  • Split ESQL functions/operators docs files #123904
  • ESQL autogenerate docs v3 #124312
  • Restructure query-languages docs files for clarity #124797
  • Rewrite Kibana docs asciidoc links to be MD links #125155
  • Additional support for docs for ES|QL operators and version-specific differentiation #125251
  • Refine ESQL docs handling of applies_to #125835
  • Kibana updates, remove links from JSON and split is-null/is-not-null #125986
• Any new features added or manual docs edited, make sure we have the correct docs in 8.x, 9.0 and 9.1:
  • docs for CHANGE_POINT (this is an entirely new command, should this not have its own doc issue?)
  • MATCH and QSTR changes (both added optional parameters map, and this is described in the versioning work)
  • TERM support (this is currently a hidden function, the docs are there but not exposed)
  • Changes to TO_UPPER and TO_LOWER (this is part of the versioning work, done but might require refinements)
  • more?

[elasticsearchmachine]

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

[elasticsearchmachine]

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

[leemthompo]

We might want to think about better supporting links in the generated code too per #113471

[marciw]

cc @Mpdreamz @reakaleek @bmorelli25 for awareness and advice! 🙏 (not urgent)

[marciw]

We also have this related issue on the docs side, including mentions of the ES|QL generated docs: https://github.com/elastic/docs-projects/issues/276

[leemthompo]

Tip

• You can get a taste of the syntax here: https://elastic.github.io/docs-builder/syntax/
• Here's a Live preview of new docs building off of the docs-content repo

Note

The bulk of the ESQL docs are in temporary repo awaiting rerouting. You'll find more syntax examples there.