[WIP] Sitemap generation

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #132 
| License       | MIT
| Doc PR        | symfony/symfony-docs#123https://github.com/symfony-cmf/symfony-cmf-docs/issues/607

Started working on that feature. That would be the task:

- [ ] implement generator for phpcr content
- [ ] create decider for content which should be published
- [x] create controller for the output
- [ ] build dynamic route generation depending on config
- [ ] general configuration and optional activating of the phpcr generator
I think We should add that, cause We have the data at this point. Would have to add some kind of sorting while creating the list (nested set idea) in the generator. the template i forgot to implement would be a generic one to override it.