[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
can you explain that this interface is for documents that can go into the sitemap. and explain the bit about using the right property name so that the default provider works?