[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:

- [x] implement generator for phpcr content
- [x] create decider for content which should be published
- [x] create controller for the output
- [x] general configuration and optional activating of the phpcr generator
don't call this `isIs`. i propose `isVisibleInSitemap`