docs-rst: Don't mangle with UTF-8 chars on LaTeX/PDF output

pdflatex doesn't accept using some UTF-8 chars, like
"equal or less than" or "equal or greater than" chars. However,
the media documents use them. So, we need to use XeLaTeX for
conversion, and a font that accepts such characters.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>