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
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REL_14_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/af2c08671bc55649301d11f5e221b9cead8386ca

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