media: docs: Add a master topic for the index file

Right now, both V4L and Digital TV are shown as main titles.

Instead, add a main title, in order to better display its
output.

While here, simplify the file, as we don't need to add
a title for the indexes, nor we need copyright info, as
everything there is GPL, as stated by the SPDX tags.

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