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 a small sentence between each example would make a lot of sense.
In particular here, you need rto explain that a true one to one (one to one + PKJC) implies that the same id is shared between the two entities and that it is not needed to keep a fk copy as the pk is also the fk