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

Documentation/conf.py

@@ -456,8 +456,15 @@ htmlhelp_basename = "TheLinuxKerneldoc"
 latex_elements = {
     # The paper size ('letterpaper' or 'a4paper').
     "papersize": "a4paper",
+    "passoptionstopackages": dedent(r"""
+        \PassOptionsToPackage{svgnames}{xcolor}
+        % Avoid encoding troubles when creating indexes
+        \PassOptionsToPackage{xindy}{language=english,codepage=utf8,noautomatic}
+    """),
     # The font size ('10pt', '11pt' or '12pt').
     "pointsize": "11pt",
+    # Needed to generate a .ind file
+    "printindex": r"\footnotesize\raggedright\printindex",
     # Latex figure (float) alignment
     # 'figure_align': 'htbp',
     # Don't mangle with UTF-8 chars