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.
> with an initial value of 2 and an allocationSize of 20, I would have expected 22 in next_val I think. was that from actually running the code?

No, if I run the code I get 42. Not sure why