Merge branch 'mauro-pdf' into docs-mw

Merge branch 'mauro-pdf' into docs-mw

Here it is the second version of the PDF series. I opted to split one of
the patches in 3, to have a clearer changelog and description.

Also, archlinux LXC image download started working again, so I added
an extra patch addressing texlive packae dependencies.

This series is taking me a way more time than antecipated.

This series as 3 goals:

1. Fix a pre-Sphinx 1.7 PDF variable that got renamed, but
   our Makefile still uses the old one that is not supported
   since Sphinx 1.7;

2. Fix broken or incomplete texlive dependencies on several
   distros;

4. "modernize" conf.py to solve font conflicts related to UTF-8
   and non-UTF fonts from [T1]{fontenc}  LaTeX package.

   Using fontenc with xelatex is problematic, as documented at

	https://www.sphinx-doc.org/en/master/latex.html

Please notice that:

- It doesn't pretend to fix all  PDF issues. It focus only at the
  above;
- there are still distros where PDF builds fail either partially
  or as a hole. On my checks, those are due to problematic
  texlive packages shipped on such distros;
- it doesn't touch/address/alter anyhing related to kfigure.py.
  as such, it doesn't touch/change/improve/drop anything with
  regards ImageMagick and/or Inkscape.