This documentation should help you a bit: https://radmatt.itch.io/godot-3-dialogue-system-documentation. I've updated the docs with a new category (see 8. File Structure).
JSON files are difficult to write by hand, so I made a software for that: https://radmatt.itch.io/dialogue-designer. It it tightly integrated with this dialogue system so there's no need to modify the files by hand.