Document-collection in Airflow

[Ben-Hodgkiss]

Overview
The document-collection dag in Airflow has been turned off since we migrated to Airflow. We need to speak with those who implemented Airflow to work out why. If we still need the dag, then we should fix it so it works like every other as part of the overnight collection run. If it's no longer required, then we should remove it from Airflow to avoid confusion going forward.