Doc: Move standalone backup section, mention -X argument

A documentation comment came in [1] causing me to review some of our backup documentation and I left the current content and location of the standalone backups was odd.  I propose to move it to a better place, under file system backups.

Adding to commitfest.

David J.

[1] https://www.postgresql.org/message-id/CAKFQuwZ%3DWxdWJ6O66yQ9dnWTLO12p7h3HpfhowCj%2B0U_bNrzdg%40mail.gmail.com