Update cmake setup to build documentation

[vkottler]

It seems to have these bootstrap options:

  --sphinx-info           build Info manual with Sphinx
  --sphinx-man            build man pages with Sphinx
  --sphinx-html           build html help with Sphinx
  --sphinx-qthelp         build qch help with Sphinx
  --sphinx-latexpdf       build PDF with Sphinx using LaTeX
  --sphinx-build=<sb>     use <sb> as the sphinx-build executable
  --sphinx-flags=<flags>  pass <flags> to sphinx-build executable

It also has some other useful options, like:

  --parallel=n            bootstrap cmake in parallel, where n is                                                                                                                                                                                              number of nodes [1]
...
  --generator=<generator> generator to use (MSYS Makefiles, Unix Makefiles,                                                                                                                                                                                    or Ninja)
  --enable-ccache         Enable ccache when building cmake

By default it looks like we don't get man pages or documentation or anything. That would be pretty useful

[vkottler]

Finished this on 88a0789a1ac08cc08ac85286e6cf0c5473127d48