For onboarding with a given unique ID, this method will find a document with a given document unique ID and will try to add that document to an onboarding.
If the process fails, this method returns 404 (Not Found) if either onboarding or document was not found, or 400 (Bad Request) if the action of adding this document to this onboarding is illegal by business logic.
Response of this endpoint is generated unique global identifier of onboarding document.