[FEATURE] Rest-Api

__Tasks:__

- [x] set up the database-model and the bundle
- [x] create a rest-api for the categories
- [x] gather feedback for my changes
- [x] submit changes to the documentation


__Informations:__

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| Tests pass?   | yes
| Fixed tickets | none
| Doc           | https://github.com/sulu-cmf/docs/blob/master/detail-specification/100-basic/DET-107-Categories.md