[media] DocBook: move DVBv3 frontend bits to a separate section

Although the recommended usage is the DVBv5 API calls, the
documentation doesn't make it clear about what's the recommended
calls and what's legacy.

So, move the legacy API bits to a separate xml, putting them into
a new section.

Please notice that more changes are needed, since some of the
bits there are cross-referenced elsewhere.
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>