Updates on Docbook information

In https://www.postgresql.org/docs/18/docguide-docbook.html it says:
"...
DocBook is maintained by the OASIS group. The [official DocBook site]
has good introductory and reference documentation and a complete
O'Reilly book for your online reading pleasure. The [FreeBSD
Documentation Project] also uses DocBook and has some good
information, including a number of style guidelines that might be
worth considering.
"
which caught my attention with:
- "DocBook is maintained by the OASIS group.": No longer true, see
https://docbook.org/
- [official DocBook site] points to
https://www.oasis-open.org/docbook/, and "has good introductory and
reference documentation and a complete O'Reilly book for your online
reading pleasure" seems not appropriate for the page
- "The [FreeBSD Documentation Project] also uses DocBook and has some
good information, including a number of style guidelines that might be
worth considering.": No longer true, it currently use asciidoc and
there are no references to docbook in the current website

I think those sentences should be replaced with:
"...
The [DocBook site] has good introductory and reference documentation,
a complete O'Reilly book for your online reading pleasure, and an
archive with previously released versions.
"

Also, in https://www.postgresql.org/docs/18/docguide-toolsets.html says:
"...
[DocBook DTD]
This is the definition of DocBook itself. We currently use version
4.5; you cannot use later or earlier versions. You need the XML
variant of the DocBook DTD, not the SGML variant.
"
where [DocBook DTD] points to https://www.oasis-open.org/docbook/ and
it is not found there. It should point to
https://archive.docbook.org/schemas/ where you can find the 4.5 DTD.

I attach a patch with the proposed changes for review / further refinement.

Thanks!