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.
>  the property order is not preserved, I wonder if that's on purpose.

I think I just copied and paste from the mongodb terminal and that was the way the document was returned.
I didn't pay attention to the order but it make sense to preserve it.