This creates the `MV_FIRST` and `MV_LAST` functions that return the
first and last values from a multivalue field. They are noops from a
single valued field. They are quite similar to `MV_MIN` and `MV_MAX`
except they work on positional data rather than relative size. That
sounds like a large distinction, but in practice our multivalued fields
are often sorted. And when they operate on sorted arrays `MV_MIN` does
*the same* thing as `MV_FIRST`.
But there are some cases where it really does matter - say you are
`SPLIT`ing something - so `MV_FIRST(SPLIT("foo;bar;baz", ";"))` gets you
`foo` like you'd expect. No sorting needed.
Relates to #103879
* 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>
