media: admin-guide: split DVB cards.rst

The DVB cards.rst file is old and outdated. It also contains
data that are contained on other cardlists. Remove the
duplicated information and split frontends and PCI cards on
separate files.

As all USB cards already have their own card lists, just drop
the old USB data there.

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