[media] doc-rst: move v4l2-dev doc to a separate file

Move the documentation for video device node creation to
a separate file.
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

Documentation/media/kapi/v4l2-core.rst

@@ -5,6 +5,7 @@ Video2Linux devices
     :maxdepth: 1
 
     v4l2-framework
+    v4l2-dev
     v4l2-controls
     v4l2-device
     v4l2-dv-timings