Start the annotations reference doc

The idea is to do something like the serializer doc: have everything in detail in the annotations reference.

- [ ] Explain that embeds use the `@Serializer\XmlRoot` for the xml element names
- [ ] Expression Language reference
- [ ] Expression Evaluator context
- [ ] Caching configuration for a non symfony app ?
- [ ] Document the existing json/xml serializers, and how to add new ones
- [ ] ConfigurationExtensionInterface