[media] doc-rst: Split v4l-core into one file per kAPI

Sphinx produce a 1:1 mapping between a rst file and an html file.

So, we need to split the kernel-doc tags on multiple documents.

A side effect is that we're now having a better name for each
section of the kAPI documentation.

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