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'm pretty sure I've copied this from the terminal, but I'll double check

On Fri, Nov 28, 2014 at 4:44 PM, Gunnar Morling <notifications@github.com>
wrote:

> In documentation/manual/src/main/asciidoc/en-US/modules/mongodb.asciidoc:
>
> > +----
> > +{
> > +  "_id" : "alex",
> > +  "name" : "Alex",
> > +  "wife" : "bea"
> > +}
> > +----
> > +
> > +Wife collection as JSON in MongoDB
> > +
> > +[source, JSON]
> > +----
> > +{
> > +  "_id" : "bea",
> > +  "name" : "Bea",
> > +  "husband" : [ "alex" ]
>
> This should be not mapped like this anymore, just the singular id value.
> That's in for MongoDB for a while, for CouchDB I sent the PR earlier today.
>
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> <https://github.com/hibernate/hibernate-ogm/pull/403/files#r21038967>.
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