docs: conf.py: fix some troubles for LaTeX output

While PDF docs work fine on RPM-based distros, it causes conflicts
on Debian & friends.

There are multiple root causes here:

- the latex_elements still resambles the one from Sphinx 1.x
times, where font configurations were stored under
"preamble". It doesn't follow the current recommended way from
Sphinx documentation:

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

- instead of setting the main font, from where other fonts are
derivated, it sets romanfont;

- "fontenc" is not set. This allows the *.tex output file to
contain an UTF-8 incompatible fontset:

\usepackage[T1]{fontenc}

Address such issues to help preventing incompatible usage of
both T1 font and UTF-8 ones that comes from DejaVu font
family.

On some distros, this even generate a LaTeX font warning about
corrupted NFSS tables like this (I got it when running it in
interactive mode):

Package: fontenc 2021/04/29 v2.0v Standard LaTeX package
LaTeX Font Info: Trying to load font information for T1+lmr on input line 11
6.
LaTeX Font Info: No file T1lmr.fd. on input line 116.

LaTeX Font Warning: Font shape `T1/lmr/m/n' undefined
(Font) using `T1/lmr/m/n' instead on input line 116.

! Corrupted NFSS tables.
wrong@fontshape ...message {Corrupted NFSS tables}
error@fontshape else let f...
l.116 ...\familydefault\seriesdefault\shapedefault

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/8104ce56a5ea3509fbb56a8163b779d38a6aaa9c.1755763127.git.mchehab+huawei@kernel.org