media: docs: update avermedia.rst contents

While this is old, now that we moved the intro part of it,
its contents seem to be valid, if we mention that we're
talking only about the BT878 support.

Update the document title accordingly and remove the obsolete
note from it.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

media: docs: split avermedia.rst contents on two files

Part of this document has nothing to do with the Avermedia
driver. It is generic to the entire subsystem. So, split it
on a separate file.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

media: docs: move user-facing docs to the admin guide

Most of the driver-specific documentation is meant to help
users of the media subsystem.

Move them to the admin-guide.

It should be noticed, however, that several of those files
are outdated and will require further work in order to make
them useful again.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>