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.
BTW nowhere in the doc have I seen a clear explanation of `AssociationStorage` vs `AssociationDocumentStorage`, why two anntoations and what they correspond to.

Speaking of, why is AssociationDocumentStorage a mongodb specific option, it seems generic, no? CC @gunnarmorling 

Speaking of 2, we use "storage" here, couldn't we use "storage" too for the k/v cache "mapping" we discussed earlier this week.