moving documentation to XML

For 9.4, I would like to tackle moving our documentation to a DocBook
XML source format and tool chain.  The issues surrounding that have been
summarized on this wiki page:
<https://wiki.postgresql.org/wiki/Switching_PostgreSQL_documentation_from_SGML_to_XML>.
 Most of these points have become no-brainers by now.  The one remaining
major issue is that the XSLT HTML build is very very slow compared to
what we currently have.  So the first step will be to address that.  I
have a few ideas that I'll try, and I might also solicit some help from
DocBook-related forums.  Anyone who wants to help, it's very easy to get
started.  In doc/src/sgml, run

make html

vs

make xslthtml

and make the latter as fast as the former.  Currently, I'm timing this
at about 1 minute versus about 11 minutes, which is clearly unacceptable.

If or when that gets fixed, the next steps would be

- use XSLT as primary method for building HTML (dump DSSSL style sheets)

- use FOP as primary method for building PDF (additional issues to solve
there)

- convert source to XML