Move and merge Explore & Analyze > SQL reference docs into Reference > Query languages > SQL docs #2944
Description
Follow up 1/2 to #2563
SQL docs pages under Explore & Analyze are in fact mostly reference documentation, and it creates a fragmented experience to have some pages in Explore & Analyze and other pages under the Reference section.
The goal of this issue is to move the SQL docs content under Explore and Analyze to the SQL section of the Reference docs, and create a consistent experience for users who want to use this language as part of their Elasticsearch use cases.
In "Explore and Analyze with Kibana", the focus is on how to leverage these Elasticsearch concepts when working with Kibana. As such, A single SQL page should remain, showing where in Kibana it makes sense to use the SQL syntax (there will be a distinct issue for rewriting that standalone SQL page in Explore and Analyze).