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.
What about collection of simple types like 

```
class GrandMother {
    @ElementCollection (or is that another annotation that is expected?)
     private List<String> grandChild;
}