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 
the page document lives in the `Model` namespace, the one in `Doctrine\Phpcr` should only be used when one is using the PHPCR-ODM.