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.
> At that point I felt like only moving that stuff to core which is guaranteed to be portable.

Hum, my comment on `DocumentStoreProperties` does not reflect this: "Note that not all properties are supported by all datastores; refer to the documentation of the specific dialect to find out the supported configuration properties" (referring to props, but it's similar to options).

Probably makes sense to decide based on the likelyhood whether it's going to be shared by the majority of stores of the family or not.