pgsql: doc: Remove dead link to NewbieDoc Docbook Guide

doc: Remove dead link to NewbieDoc Docbook Guide

The link returns 404 and no replacement is available in the project
on Sourceforge where the content once was. Since we already link to
resources for both beginner and experienced docs hackers, remove the
the dead link.

Backpatch to all supported versions as the link was added in 8.1.

Author: Daniel Gustafsson <daniel(at)yesql(dot)se>
Reviewed-by: Magnus Hagander <magnus(at)hagander(dot)net>
Reviewed-by: Michael Paquier <michael(at)paquier(dot)xyz>
Reported-by: jian he <jian(dot)universality(at)gmail(dot)com>
Discussion: https://postgr.es/m/CACJufxH=YzQPDOe+2WuYZ7seD-BOyjCBmP6JiErpoSiVZWDRnw@mail.gmail.com
Backpatch-through: 13

Branch
------
REL_13_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/0a277da651696304c3ffd34eb67cc7a6737b4106

Modified Files
--------------
doc/src/sgml/docguide.sgml | 4 +---
1 file changed, 1 insertion(+), 3 deletions(-)