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_17_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/805b85027ad7bc4422ef5ea4041fd3de8e175459

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