[DOCS] Document EQL support in Elasticsearch

[jrodewig]

This is a meta issue to track the progress of documentation efforts for EQL support in Elasticsearch.
To monitor ongoing development, see #49581.

High-level content plan

• Top-level EQL page (intro + nav) [DOCS] Add top-level EQL docs page. Adds EQL requirements page. #51334
• EQL requirements [DOCS] Add top-level EQL docs page. Adds EQL requirements page. #51334
• Run an EQL search [DOCS] Add basic EQL search tutorial docs #51574
  • Add a Beats tip [DOCS] Adds Beats tip to EQL search docs #53292
  • Specify timestamp and event type fields [DOCS] Add parameter examples to EQL search tutorial #52953
  • Filter using query DSL [DOCS] Add parameter examples to EQL search tutorial #52953
  • Async search [DOCS] EQL: Document async search submits #56704
• EQL syntax reference [DOCS] Add EQL syntax page #51821
  • Basic syntax (https://eql.readthedocs.io/en/latest/query-guide/index.html) [DOCS] Add EQL syntax page #51821
• EQL function reference
  • add [DOCS] EQL: Document math functions #55810
  • between [DOCS] EQL: Document between function #54950
  • cidrMatch [DOCS] EQL: Document cidrMatch function #54216
  • concat [DOCS] EQL: Document concat function #56239
  • divide [DOCS] EQL: Document math functions #55810
  • endsWith [DOCS] EQL: Document endsWith function #54521
  • indexOf [DOCS] EQL: Document indexOf function #55071
  • length [DOCS] EQL: Document length function #54225
  • match [DOCS] EQL: Document match function #56134
  • modulo [DOCS] EQL: Document math functions #55810
  • multiply [DOCS] EQL: Document math functions #55810
  • number[DOCS] EQL: Document number function #56770
  • startsWith [DOCS] EQL: Document startsWith function #54518
  • string [DOCS] EQL: Document string function #55086
  • stringContains [DOCS] EQL: Document stringContains function #54968
  • substring [7.x] [DOCS] EQL: Document substring function (#53867) #54203
  • subtract [DOCS] EQL: Document math functions #55810
  • wildcard [DOCS] EQL: Document wildcard function #54086
• EQL pipe reference [DOCS] EQL: Document head and tail pipes #58673
  • head [DOCS] EQL: Document head and tail pipes #58673
  • tail [DOCS] EQL: Document head and tail pipes #58673
• Limitations (EQL features not supported in ES) [DOCS] Add EQL limitations page #52001
• EQL search API documentation [DOCS] Document EQL search REST API #52384

Cleanup tasks

• Remove/swap dev admonitions [DOCS] EQL: Prepare docs for release #59259
• Remove ifdef statements for including docs (cf. [DOCS] Include docs on permanently unreleased branches only #51743, [DOCS] Document EQL search REST API #52384) [DOCS] EQL: Prepare docs for release #59259

[elasticmachine]

Pinging @elastic/es-docs (>docs)

[elasticmachine]

Pinging @elastic/es-search (:Search/EQL)