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.
Yes, they are. It's a thing which made me wonder every now and then. Do you think it's good that way? It's the reason we cannot store the key more nicely (in case it's not composed), as we otherwise wouldn't know from which side an entry originates.

Should we have yet another strategy with two distinct collections? It would allow for a nicer key representation, but then I dislike all this sort of options.