Doc: add see-also references to CREATE PUBLICATION.

Doc: add see-also references to CREATE PUBLICATION.

The "See also" section on the reference page for CREATE PUBLICATION
didn't match the cross references on CREATE SUBSCRIPTION and their
ALTER counterparts. Fixed by adding an xref to the CREATE and ALTER
SUBSCRIPTION pages.  Backpatch down to v10 where CREATE PUBLICATION
was introduced.

Author: Peter Smith <smithpb2250@gmail.com>
Reviewed-by: Masahiko Sawada <sawada.mshk@gmail.com>
Discussion: https://postgr.es/m/CAHut+PvGWd3-Ktn96c-z6uq-8TGVVP=TPOkEovkEfntoo2mRhw@mail.gmail.com
Backpatch-through: 10