OGM-606 JPA Examples in documentation

I've decided to send a new pull request for https://hibernate.atlassian.net/browse/OGM-606
as a follow up of  https://github.com/hibernate/hibernate-ogm/pull/403.

I've applied the remarks.
I've also added a commit for the null values.

@emmanuelbernard 
I mistakenly squashed your commits while fixing conflicts. Even if the commits are not in the pull the changes are included.

About this comment:
>I would add an example of @OrderColumn for List and one example of a embeddable id + a map or >@OrderColumn to show how the id and the id coexist.

In the documentation there is now an example for all of this use cases but I haven't added one the inlcude all of them...is that what you are asking?

Things that are missing are:
* New mapping for EhCache and Infinispan
* Small introduction to the examples
* Add examples with the new mapping for MongoDB EmbeddedId

I would keep the corresponding issue open and update it with the missing things. WDYT?