[media] doc-rst: move bttv documentation to bttv.rst file

There were several files under Documentation/video4linux/bttv.

Instead of simply copying them to the rst folder, I opted to
merge into a single document and adjust the headers to
adjust the section levels and fix the cards tables.

There are two exceptions on the merge:

- The Tuners were renamed as a separate document, as they
  describe a separate driver;

- I removed the PROBLEMS section. It describes problems with
  the very first generation of 3D boards (Mistique/S3).
  It sounds very unlikely that someone would still need to
  install a bttv board on such hardware. Also, it is not
  very well written, IMHO.

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