scripts: sphinx-pre-install: add more generic checkers on a class

scripts: sphinx-pre-install: add more generic checkers on a class

Better organize the code by moving the more generic methods
to MissingCheckers. Such class contain only binary and package
dependent missing checkers, but no distro-specific data or code.

All distro-specific data/code remains at SphinxDependencyChecker
class.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/11a252fe816bd7c85583d26ade0666eb2b481bf0.1754992972.git.mchehab+huawei@kernel.org