OGM-673 OMG-690 OGM-683

This PR takes all the previous documentation improvements of today.

* each datastore is top level
* rework query chapter
* move mapping and query before the datastore chapters
* various documentation improvements
* mention embedded objects and @Column for mongodb

Note that OGM-603 still needs to be done. I need to add an API section between mapping and query.
I feel like an example with AttributeOverride and an example of what happens without the dot in the column name would help.