Update SELinux links in sepgsql documentation

Update SELinux links in sepgsql documentation

The SELinux FAQ and User Guide links has been redirecting from the
URL in our documentation for a very long time (since 2011 according
to archive.org), but the redirect is no longer maintained and they
now return a 404 since some time back.  Fix by replacing the links
with the targets of the redirect.

This is a follow-up to commit cc1853b3 which updated the links in
version 10 through master, which albeit different from this one
were also yielding 404.

Discussion: https://postgr.es/m/AF6E0911-CA4A-4B5B-91AB-51E388674569@yesql.se