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.
> Speaking of 2, we use "storage" here, couldn't we use "storage" too for the k/v cache "mapping" we discussed earlier this week.

You mean the name of the K/V option, e.g. `CacheStorage`? Yes, that sounds good. Or do you mean to provide a shared option somehow?