docs: admin-guide: add a features list

Add a feature list matrix at the admin-guide.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/f6c1e366fbc7ce1c9c94c7dc6c7852c6377cc0be.1606748711.git.mchehab+huawei@kernel.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>