[media] v4l2-dev: add cross-references and improve markup

Add cross-references for the functions/structs and add
the markup tags to improve its display.
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

Documentation/media/kapi/v4l2-videobuf.rst

+.. _vb_framework:
+
 Videobuf Framework
 ==================
 

Documentation/media/kapi/v4l2-videobuf2.rst

+.. _vb2_framework:
+
 V4L2 videobuf2 kAPI
 ^^^^^^^^^^^^^^^^^^^