doc-rst: add an option to ignore DocBooks when generating docs

Sometimes, we want to do a partial build, instead of building
everything. However, right now, if one wants to build just
Sphinx books, it will build also the DocBooks.

Add an option to allow to ignore all DocBooks when building
documentation.

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