This adds the documentation for BUCKET as a grouping function and the
addition of the "direct" invocation mode providing a span (in addition
to the auto mode).
This renames the function AUTO_BUCKET to just BUCKET.
It also removes the experimental tagging of the function in the docs, making it generally available.
* 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>
