Docs: Allow Generation of LaTeX PDF

This MR is tracking the progress of building a PDF of the different guides (or one for all).

• Pending pydata-sphinx-theme fix https://github.com/pydata/pydata-sphinx-theme/pull/2076

• TOC messed up, "Key features" appears as chapter, everything else as section therein

• Find a way to drop parts of the build (news, landing page, potentially Doxygen reference?)

• Possibly allow to build separate targets for the three guides

• Fix issue with Pygment generation:

  Writing evaluated template result to /home/simonspa/software/constellation-website/build/latex/sphinxmessages.sty
  
  Extension error (pydata_sphinx_theme.pygments):
  Handler <function overwrite_pygments_css at 0x757db81491c0> for event 'build-finished' threw an exception (exception: [Errno 2] No such file  or directory: '/home/simonspa/software/constellation-website/build/latex/_static/pygments.css')
  make: *** [Makefile:25: latexpdf] Error 1
  make: Leaving directory '/home/simonspa/software/constellation/docs'

  (copying _static from html generation works)