[media] doc-rst: better organize the media books

The uAPI book has 5 parts, but they lost numeration after
conversion to rst. Manually number those parts, and make
the main index with 1 depth, to only show the parts and
the annexes.

At each part, use :maxwidth: 5, in order to show a more
complete index.

While here, fix the cross-references between different
books.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>