media: admin-guide: add a generic building guide

Several of the existing documents under the media admin-guide
contain build procedures.

Add an specific chapter describing it. This document was
partially inspired on the modifications I made to the bttv.rst
file.
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

Documentation/admin-guide/media/index.rst

@@ -35,6 +35,7 @@ Video4Linux (V4L)  driver-specific documentation
 	:numbered:
 
 	intro
+	building
 
 	cardlist