[media] rename v4l2-framework.rst to v4l2-intro.rst

Now that the only remaining chapters at v4l2-framework are
the introduction ones, let' s rename the file.
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

Documentation/media/kapi/v4l2-core.rst

@@ -4,7 +4,7 @@ Video2Linux devices
 .. toctree::
     :maxdepth: 1
 
-    v4l2-framework
+    v4l2-intro
     v4l2-dev
     v4l2-controls
     v4l2-device