[media] doc-rst: add documentation for bttv driver

Convert it to ReST and add it to media/v4l-drivers book.
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

Documentation/media/v4l-drivers/index.rst

@@ -22,6 +22,7 @@ License".
 	fourcc
 	v4l-with-ir
 	cardlist
+	bttv
 	cafe_ccic
 	cpia2
 	cx18