[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 i would call this thing SitemapRouteProvider and not "Generator". we don't care how those routes come into existence, but we do want that the thing gives us routes. so provider would be the right term imho