[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
do we even need a html representation? what is the use case?
this is not suitable for the cms user site map which is structured and nicely curated. if anything, we would want some "show in sitemap" flag on routes - which can be managed with the route attributes if needed.