On iOS, I often use iA Writer to edit MarkDown documents. It would be wonderful it we could configure the “Edit” button to – depending on the file type – edit files within DTTG itself, or open files in an external app (which would then save the files in DTTG when you’re done editing).
Unfortunately not is not possible on iOS so easily. All “Open In” activity has to go through the file provider extension mechanism which is invoked not from DEVONthink To Go but from the external editor.