documentation is now XML

The documentation sources are now DocBook XML, not SGML. (The files are
still named *.sgml. That's something to think about separately.)

Besides the recent change to require full end tags, which is compatible
between SGML and XML, being XML now requires empty-element tags to use
the XML syntax like <xref linkend="foo"/>. When backpatching
documentation changes, the extra slash needs to be removed, however.
(The documentation build in backbranches will error if that is not done.)

For the required tools, there are no new requirements, but you can drop
docbook-sgml and opensp. The instructions at
<https://www.postgresql.org/docs/devel/static/docguide-toolsets.html>
will be up-to-date once built.

--
Peter Eisentraut http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services