Restructure ES|QL docs (elastic#100806)

* 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>

docs/reference/esql/esql-commands.asciidoc

@@ -0,0 +1,17 @@
+[[esql-commands]]
+== {esql} commands
+
+++++
+<titleabbrev>Commands</titleabbrev>
+++++
+
+{esql} provides a comprehensive set of source and processing commands:
+
+<<esql-source-commands>>::
+include::source-commands/esql-source-commands.asciidoc[tag=list]
+
+<<esql-processing-commands>>::
+include::processing-commands/esql-processing-commands.asciidoc[tag=list]
+
+include::source-commands/esql-source-commands.asciidoc[]
+include::processing-commands/esql-processing-commands.asciidoc[]

docs/reference/esql/esql-functions-operators.asciidoc

@@ -0,0 +1,42 @@
+[[esql-functions-operators]]
+== {esql} functions and operators
+
+++++
+<titleabbrev>Functions and operators</titleabbrev>
+++++
+
+{esql} provides a comprehensive set of functions and operators:
+
+[[esql-functions]]
+<<esql-agg-functions>>::
+include::functions/aggregation-functions.asciidoc[tag=list]
+
+<<esql-math-functions>>::
+include::functions/math-functions.asciidoc[tag=list]
+
+<<esql-string-functions>>::
+include::functions/string-functions.asciidoc[tag=list]
+
+<<esql-date-time-functions>>::
+include::functions/date-time-functions.asciidoc[tag=list]
+
+<<esql-type-conversion-functions>>::
+include::functions/type-conversion-functions.asciidoc[tag=list]
+
+<<esql-conditional-functions-and-expressions>>::
+include::functions/conditional-functions-and-expressions.asciidoc[tag=list]
+
+<<esql-mv-functions>>::
+include::functions/mv-functions.asciidoc[tag=list]
+
+<<esql-operators>>::
+include::functions/operators.asciidoc[tag=list]
+
+include::functions/aggregation-functions.asciidoc[]
+include::functions/math-functions.asciidoc[]
+include::functions/string-functions.asciidoc[]
+include::functions/date-time-functions.asciidoc[]
+include::functions/type-conversion-functions.asciidoc[]
+include::functions/conditional-functions-and-expressions.asciidoc[]
+include::functions/mv-functions.asciidoc[]
+include::functions/operators.asciidoc[]

docs/reference/esql/esql-get-started.asciidoc

@@ -0,0 +1,8 @@
+[[esql-getting-started]]
+== Getting started with {esql}
+
+++++
+<titleabbrev>Getting started</titleabbrev>
+++++
+
+coming::[8.11]

docs/reference/esql/esql-kibana.asciidoc

@@ -0,0 +1,8 @@
+[[esql-kibana]]
+== Using {esql} in {kib}
+
+++++
+<titleabbrev>Kibana</titleabbrev>
+++++
+
+coming::[8.11]

docs/reference/esql/esql-language.asciidoc

@@ -0,0 +1,16 @@
+[[esql-language]]
+== {esql} language
+
+++++
+<titleabbrev>{esql} language</titleabbrev>
+++++
+
+Detailed information about the {esql} language:
+
+* <<esql-syntax>>
+* <<esql-multivalued-fields>>
+* <<esql-metadata-fields>>
+
+include::esql-syntax.asciidoc[]
+include::multivalued-fields.asciidoc[]
+include::metadata-fields.asciidoc[]

docs/reference/esql/esql-limitations.asciidoc

@@ -0,0 +1,32 @@
+[[esql-limitations]]
+== {esql} limitations
+
+++++
+<titleabbrev>Limitations</titleabbrev>
+++++
+
+[discrete]
+[[esql-supported-types]]
+=== Supported types
+
+* {esql} currently supports the following <<mapping-types,field types>>:
+
+** `alias`
+** `boolean`
+** `date`
+** `double` (`float`, `half_float`, `scaled_float` are represented as `double`)
+** `ip`
+** `keyword` family including `keyword`, `constant_keyword`, and `wildcard`
+** `int` (`short` and `byte` are represented as `int`)
+** `long`
+** `null`
+** `text`
+** `unsigned_long`
+** `version`
+
+[discrete]
+[[esql-max-rows]]
+=== 10,000 row maximum
+
+A single query will not return more than 10,000 rows, regardless of the
+`LIMIT` command's value.

docs/reference/esql/esql-query-api.asciidoc

@@ -0,0 +1,84 @@
+[[esql-query-api]]
+== {esql} query API
+++++
+<titleabbrev>{esql} query API</titleabbrev>
+++++
+
+Returns search results for an <<esql,ES|QL ({es} query language)>> query.
+
+include::index.asciidoc[tag=esql-query-api-example]
+
+[discrete]
+[[esql-query-api-request]]
+=== {api-request-title}
+
+`POST _query`
+
+[discrete]
+[[esql-query-api-prereqs]]
+=== {api-prereq-title}
+
+* If the {es} {security-features} are enabled, you must have the `read`
+<<privileges-list-indices,index privilege>> for the data stream, index,
+or alias you search.
+
+[discrete]
+[[esql-query-api-query-params]]
+=== {api-query-parms-title}
+
+`delimiter`::
+(Optional, string) Separator for CSV results. Defaults to `,`. The API only
+supports this parameter for CSV responses.
+
+`format`::
+(Optional, string) Format for the response. For valid values, refer to
+<<esql-rest-format>>.
++
+You can also specify a format using the `Accept` HTTP header. If you specify
+both this parameter and the `Accept` HTTP header, this parameter takes
+precedence.
+
+[discrete]
+[role="child_attributes"]
+[[esql-query-api-request-body]]
+=== {api-request-body-title}
+
+`columnar`::
+(Optional, Boolean) If `true`, returns results in a columnar format. Defaults to
+`false`. The API only supports this parameter for CBOR, JSON, SMILE, and YAML
+responses. See <<esql-rest-columnar>>.
+
+`params`::
+(Optional, array) Values for parameters in the `query`. For syntax, refer to
+<<esql-rest-params>>.
+
+`query`::
+(Required, object) {esql} query to run. For syntax, refer to <<esql-syntax>>.
+
+[[esql-search-api-time-zone]]
+`time_zone`::
+(Optional, string) ISO-8601 time zone ID for the search. Several {esql}
+date/time functions use this time zone. Defaults to `Z` (UTC).
+
+[discrete]
+[role="child_attributes"]
+[[esql-query-api-response-body]]
+=== {api-response-body-title}
+
+`columns`::
+(array of objects)
+Column headings for the search results. Each object is a column.
++
+.Properties of `columns` objects
+[%collapsible%open]
+====
+`name`::
+(string) Name of the column.
+
+`type`::
+(string) Data type for the column.
+====
+
+`rows`::
+(array of arrays)
+Values for the search results.