[media] add DVB documentation to Sphinx

Now that all DVB files got converted, add it to Sphinx
build.
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

Documentation/index.rst

@@ -16,6 +16,7 @@ Contents:
    kernel-documentation
    media/media_uapi
    media/media_drivers
+   media/dvb-drivers/index
 
 Indices and tables
 ==================