update paths in SQL files to point to reference content (#3270)

Linked to #2944
This PR retains only a single page for SQL in Explore & Analyze and
creates redirects to reference content for all files under Explore &
Analyze -> SQL. All files under SQL have been moved to Reference content
in another PR: elastic/elasticsearch#135675
The single retained page for SQL will be revamped in another issue:
#2945

---------

Co-authored-by: Liam Thompson <[email protected]>
Co-authored-by: Florent Le Borgne <[email protected]>

Author: 3 people