Update SQL reference in user guide to cover all supported syntax

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
The docs at file:///home/andy/git/apache/arrow-datafusion/docs/build/html/user-guide/sql/select.html do not show that we support CTEs and subqueries.

Describe the solution you'd like
Document all SELECT syntax that we support

Describe alternatives you've considered
None