refactor route provider example

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | https://github.com/symfony-cmf/symfony-cmf-docs/issues/421

So... 
started to refactor the example for the route provider. It isn't ready. Got some open issues:
- just injecting the document manager instead of the document registry cause it is a very specific 
example?
- adding the service config for yml and php
- little refactorings in text