docs: conf.py: extra cleanups and fixes

Makes it more adehent with modern Sphinx LaTeX build setup as
defined at:

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

There, it suggests:

1) to add: 'printindex'

“printindex” call, the last thing in the file. Override if you want to generate
the index differently, append some content after the index, or change the font.
As LaTeX uses two-column mode for the index it is often advisable to set this
key to r'\footnotesize\raggedright\printindex'.

This indeed solved a corner case on a distro where the index was not properly
generated.

2) to add at the main example:
\PassOptionsToPackage{svgnames}{xcolor}

3) I got a corner case on one of the distros was using xindy to produce
indexes. This ended causing the build logic to incorretly try to use
T1 fontenc, which is not UTF-8 compatible.

This patch adds:

\PassOptionsToPackage{xindy}{language=english,codepage=utf8,noautomatic}

to cover such case. It should be noticed that, as the config doesn't
have \usepackage{xindy}, this will be used only if latexmk ends
using xindy.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/08d16c2ad817910eb5606842f776d3f77d83282f.1755763127.git.mchehab+huawei@kernel.org