move sonata reference documentation to sonata

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | #285

moving all sonata admin reference doc into one place. i suggest we still keep the tutorials and things here, as they are very cmf specific .

The removed doc appears here: https://github.com/sonata-project/SonataDoctrinePhpcrAdminBundle/pull/241