This adds some docs to the top of `docs.csv-spec` and
`docs-IT_tests_only.csv-spec` telling folks not to add more stuff there
and instead put new examples into whatever files they line up with. It
also shifts some things out of the file to "prime the pump" on cleaning
it up.
* 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
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Co-authored-by: Elastic Machine <elasticmachine@users.noreply.github.com>