media: docs: uAPI: dvb/decoder: completing the documentation

The existing documentation of the legacy DVB Decoder API was incomplete.

Revising the documentation, adding missing parts and arranging the
documentation files new.

This patch contains the new index file and links to it.

Link: https://lore.kernel.org/linux-media/20240128233249.32794-2-herdler@nurfuerspam.de


Signed-off-by: Stefan Herdler <herdler@nurfuerspam.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>