title: Custom menu in Sonata (Symfony) without creating a new entity
body: <p>I'm new to stackoverflow (it's my first post), and new to Symfony2 and SonataAdmin (a bundle) too, and I have a little problem (I searched, but I cant find a working solution..).</p>
  
  <p>I've created a few entities and they all appear on the left of the dashboard, in the menu, and this is working fine.</p>
  
  <p>But what I want to do is add tabs without creating entity, and when you click on this menu tab, only the content of the right to be changed ! Is that possible? Thank you</p>