ContentType: Block

__Beschreibung__:
Content Blocks ist eine multiplizierbare Gruppierung von Content Types

__Checklist__:

- [x] test coverage
- [x] finish the code
- [ ] gather feedback for my changes
- [x] submit changes to the documentation


| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| Tests pass?   | none
| Fixed tickets | TODO
| Doc           | https://github.com/sulu-cmf/docs/blob/master/detail-specification/300-webspaces/DET-301-template-architecture.md#template-example