media: docs: split avermedia.rst contents on two files

Part of this document has nothing to do with the Avermedia
driver. It is generic to the entire subsystem. So, split it
on a separate file.
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

Documentation/admin-guide/media/index.rst

@@ -78,6 +78,8 @@ Linux Digital TV driver-specific documentation
 	ci
 	faq
 
+	dvb_intro
+
 	avermedia
 	bt8xx
 	lmedm04