doc: Add rst2html support for building HTML docs

doc: Add rst2html support for building HTML docs

We already fall back to rst2man if Sphinx isn't available for building
the man pages.  With this commit we'll fall back to rst2html for
building the HTML docs too.  The only tricky bit here is that
HAVE_SPHINX explicitly checks for sphinx.writers.manpage.  I'm just
assuming sphinx.writers.html exists, to avoid adding a separate
SPHINX_HTML variable.  However, the HTML builder is Sphinx's default,
so it's hard to imagine it not being installed :p.  Perhaps it would
be better to drop the HAVE_* settings and SPHINXBUILD to use:

* SPHINX2MAN empty for not-available, otherwise path to script that
  can handle '-b manpage'
* SPHINX2HTML empty for not-available, otherwise path to script that
  can handle '-b html'
* RST2MAN empty for not-available, otherwise path to script
* RST2HTML empty for not-available, otherwise path to script

I'm sticking with the less intrusive change for now, but I'm happy
with either approach.