Add documentation for rendering simple cms pages in menu.

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes #334 #90
| Applies to    | ~1.0
| Fixed tickets | #334 
also add a PHP example:

````rst
.. configuration-block::

    .. code-block:: jinja

        {{ knp_menu_render('/cms/simple/mypage') }}

    .. code-block:: html+php

        <?php echo $view['knp_menu']->render('/cms/simple/mypage') ?>
````