This modifies the ESQL test infrastructure to generate more of the
documentation for functions. It generates the *Description* section, the
*Examples* section, and the *Parameters* section as separate files so we
can use them as needed. It also generates a `layout` file that's just
a guess as to how to render the whole thing. In some cases it'll work
and we can use that instead of hand maintaining a "top level"
description file for the function.
Most newly generated files are unused. We have to chose to pick them up
by replacing the sections we were manually maintaining with an include
of the generated section. Or by replacing the entire hand maintained
file with the generated top level file.
Relates to #104247
This adds a tiny blurb for each operator to the docs with a railroad
diagram of the operator's syntax and a table of the input and output
types. This also fixes the tests to correctly generate the tables for
operators.
* Break out 'Limitations' into separate page
* Add REST API docs
* Restructure commands, functions, and operators refs
* Add placeholder for getting started guide
* Group 'Syntax', 'Metafields', and 'MV fields' under 'Language'
* Add placeholder for Kibana page
* Add link from landing page
* Apply uniform formatting to ACOS, CASE, and DATE_PARSE function refs
* Reword default LIMIT
* Add support for COUNT(*)
* Move 'Commands' and 'Functions and operators' to individual pages
---------
Co-authored-by: Elastic Machine <elasticmachine@users.noreply.github.com>
