[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
or would this return an array of the thing currently called UrlElement? then we should call it UrlGenerator and the method generateUrls or getUrls.

otherwise we would need to generate the routes later, which would be a problem when routes expect required parameters.