Fix documentation and move contextual cache into SonataCacheBundle

I've added some doc and moved the `Contextual cache` part to `SonataCacheBundle` as it has been moved from `SonataPageBundle` to `SonataCacheBundle` in the very first commit of `SonataCacheBundle` (here: https://github.com/sonata-project/SonataCacheBundle/commit/fdf568fd07af0d1252a40e0079300dfdfbd905d9).