[media] cx2341x.rst: add contents of README.vbi

Finally, adds the content of README.vbi at cx2341x.rst after
its conversion to ReST format.

Now, add information about this chipset and its driver is
inside a single chapter at the media/v4l-drivers book.

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