OGM-606 JPA mapping example in documentation

https://hibernate.atlassian.net/browse/OGM-606

This is a first draft of the MongoDB documentation.

I've probably forgotten some examples and I'm sure there are some errors but I think it is a good starting point to discuss how to improve/change the documentation.
> Speaking of, why is AssociationDocumentStorage a mongodb specific option

It only works with MongoDB, but not CouchDB. In Couch everything is one large bucket, so `COLLECTION_PER_ASSOCIATION` cannot be mapped. Of course it may be mappable with other document stores if they have the notion of "tables" or "collections" etc. At that point I felt like only moving that stuff to core which is guaranteed to be portable. Do you think we should re-consider it?