OGM-682 [Neo4j] Remove unused image
OGM-682 [Neo4j] Update embeddable node paragraph in doc
OGM-682 [Neo4j] Update embedded example image
OGM-678 Remove AssociationOperationType.PUT_NULL operation

It was never used and is quite doubtful it would be necessary.
I added a Contracts.assertNotNull on association.put to prevent setting
the value to null.