DRIVERS-1939:
Added new $documents stage to feed documents into a pipeline. Syntax is in here: https://docs.google.com/document/d/1yRcAcMUJysh4B-OMUDp2XsjgPgozYj7Mo2PwfepTC_U/edit#heading=h.wlhemcw4t2ys
This ticket was split from DRIVERS-1939, please see that ticket for a detailed description.