docs-rst: automatically convert Graphviz and SVG images

This patch brings scalable figure, image handling and a concept to
embed *render* markups:

* DOT (http://www.graphviz.org

)
* SVG

For image handling use the 'image' replacement::

    .. kernel-image::  svg_image.svg
       :alt:    simple SVG image

For figure handling use the 'figure' replacement::

    .. kernel-figure::  svg_image.svg
       :alt:    simple SVG image

       SVG image example

Embed *render* markups (or languages) like Graphviz's **DOT** is
provided by the *render* directive.::

  .. kernel-render:: DOT
     :alt: foobar digraph
     :caption: Embedded **DOT** (Graphviz) code.

     digraph foo {
      "bar" -> "baz";
     }

The *render* directive is a concept to integrate *render* markups and
languages, yet supported markups:

* DOT: render embedded Graphviz's **DOT**
* SVG: render embedded Scalable Vector Graphics (**SVG**)

Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Tested-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Tested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com> (v2 - v5)
Signed-off-by: Markus Heiser <markus.heiser@darmarit.de> (v1, v6)
Signed-off-by: Jonathan Corbet <corbet@lwn.net>