title: Symfony sonata admin bundle without entity
body: <p>Is there the way to use sonata admin bundle without entity - so without doctrine?</p>
  
  <p>I need to list files on some directory, this list is not stored in database.</p>
  
  <p>The first approach I tried was to declare my own Model manager, but there are some places which require doctrine queries etc.</p>
  
  <p>I know, that there is <a href="https://github.com/doctrine/mongodb-odm" rel="noreferrer">document manager for mongo</a> and <a href="https://github.com/sonata-project/SonataDoctrineMongoDBAdminBundle" rel="noreferrer">sonata mongo admin</a>, but in my case it's not so helpful.</p>
  
  <p>Any ideas?</p>