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 think it requires to use `ObjectId` or use `@Type(type="objectid")`, correct? This should be mentioned if that is the case.