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 am not happy with the behavior of `AUTO` in mongo (different between the "new id generators" and the "old id generators". Can you mention in the doc that using AUTO in MongoDB is not recommended and experimental.
Let's open a JIRA to track this and mention it in the doc.