doc-rst: v4l2: numerate the V4L2 chapters

The Video4Linux documentation is big. We want to numerate the
items, as it makes easier to reference an item while discussing
the document on meetings.
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

Documentation/linux_tv/media/v4l/v4l2.rst

@@ -10,7 +10,9 @@ Video for Linux Two API Specification
 **Revision 4.5**
 
 .. toctree::
+    :numbered:
     :maxdepth: 1
+    :caption: Table of Contents
 
     common
     pixfmt