Stop recommending auto-download of DTD files, and indeed disable it.

Stop recommending auto-download of DTD files, and indeed disable it.

It appears no longer possible to build the SGML docs without a local
installation of the DocBook DTD, because sourceforge.net now only
permits HTTPS access, and no common version of xsltproc supports that.
Hence, remove the bits of our documentation suggesting that that's
possible or useful.

In fact, we might as well add the --nonet option to the build recipes
automatically, for a bit of extra security.

Also fix our documentation-tool-installation recipes for macOS to
ensure that xmllint and xsltproc are pulled in from MacPorts or
Homebrew.  The previous recipes assumed you could use the
Apple-supplied versions of these tools; which still works, except that
you'd need to set an environment variable to ensure that they would
find DTD files provided by those package managers.  Simpler and easier
to just recommend pulling in the additional packages.

In HEAD, also document how to build docs using Meson, and adjust
"ninja docs" to just build the HTML docs, for consistency with the
default behavior of doc/src/sgml/Makefile.

In a fit of neatnik-ism, I also made the ordering of the package
lists match the order in which the tools are described at the head
of the appendix.

Aleksander Alekseev, Peter Eisentraut, Tom Lane

Discussion: https://postgr.es/m/CAJ7c6TO8Aro2nxg=EQsVGiSDe-TstP4EsSvDHd7DSRsP40PgGA@mail.gmail.com