RouteProvider for Portal Content

Implements a routing provider for loading sulu content

- [x] test coverage
- [x] finish the code
- [x] gather feedback for my changes
- [x] submit changes to the documentation


| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| Tests pass?   | yes
| Fixed tickets | no
| Doc           | https://github.com/sulu-cmf/docs/blob/master/detail-specification/000-foundation/DET-004-Routing.md