media: dtv-core.rst: split into multiple files

Instead of document all kAPI into a single file, split it
on multiple ones. That makes easier to maintain each part.

As a side effect, it will produce multiple html pages, with
is a good idea.

No changes at the text. Just some chapter levels changed.

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