BUG #17238: Link to "Using EXPLAIN" in the "EXPLAIN" documentation

The following bug has been logged on the website:

Bug reference:      17238
Logged by:          Tie Tyt
Email address:      tietyt@gmail.com
PostgreSQL version: 14.0
Operating system:   Mac OS Big Sur
Description:

"Explain" documentation:
https://www.postgresql.org/docs/current/sql-explain.html
"Using EXPLAIN" documentation:
https://www.postgresql.org/docs/current/using-explain.html

The "Explain" documentation does not have a link to "Using EXPLAIN" anywhere
in its documentation. These are highly related, and likely an interesting
followup topic for anyone reading the former. Therefore, I think the explain
documentation should link to the "Using EXPLAIN" documentation -- maybe as a
"to learn more..." link.