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

doc/src/sgml/docguide.sgml

@@ -60,9 +60,7 @@
    maintained by the <ulink url="https://www.oasis-open.org">
    OASIS group</ulink>.  The <ulink url="https://www.oasis-open.org/docbook/">
    official DocBook site</ulink> has good introductory and reference documentation and
-   a complete O'Reilly book for your online reading pleasure.  The
-   <ulink url="http://newbiedoc.sourceforge.net/metadoc/docbook-guide.html">
-   NewbieDoc Docbook Guide</ulink> is very helpful for beginners.
+   a complete O'Reilly book for your online reading pleasure.
    The <ulink url="https://www.freebsd.org/docproj/">
    FreeBSD Documentation Project</ulink> also uses DocBook and has some good
    information, including a number of style guidelines that might be