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.

> I don't think what you are saying is correct. for a collection (to-many association), using @JoinColumn would have quite adverse consequences. CC @gunnarmorling

Right, it works for one-to-one like in the example but it does not work for one-to-many