sphinx.rst: better organize the documentation about PDF build

Instead of having it on just one note, add a separate section.

This way, we could later improve it, providing a better
guide about the needed steps for PDF builds.

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