[media] doc-rst: increase depth of the main index

It is useful to have an index with all the book contents somewhere,
as it makes easier to seek for something. So, increase maxdepth
to 5 for the main index at the beginning of the book.

While here, remove the genindex content, as it is bogus.

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