Build doctrine media entities

__Tasks:__

- [ ] test coverage
- [x] gather feedback for my changes
- [x] submit changes to the documentation


__Informations:__

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| Tests pass?   | none
| Fixed tickets | none
| Doc           |  https://github.com/sulu-cmf/docs/blob/master/detail-specification/500-assets/DET-501.md