With the help of this answer I've been able to view the specific .json file representing my notebook. This answers the Q of where zeppelin notebooks exist in Docker images; leaving unanswered the question of where they exist in the Windows filesystem when NOT using Docker.
If using Docker (as recommended by the Zeppelin docs) this is the recipe for finding a notebook file:
# first steps are taken in Powershell
docker ps # displays an image id for use in next line
docker commit <image_id_here> mysnapshot
docker run -t -i mysnapshot /bin/bash
# now in bash inside the Docker image
root@91f4bf850583:/zeppelin# ll notebook/2E6D1WBGT/note.json
... so (if using Docker) the best way to version-control a notebook is by using Docker versioning. This is quite different to Jupyter's approach where one can run individual .ipynb notebooks locally and version-control each of them. Having answered my own question I feel greatly more informed about Docker and the differences between Jupyter vs Zeppelin. Very curious to know if anyone can solve the original question of where notebooks are stored if running in Windows WITHOUT Docker