Overhaul how this repository is managed

[rra]

• Rename the master branch to main
• Remove the separate prod branch and instead run mobu tests from main
• Review all the notebooks and reconsider the structure, ideally associating notebooks with services they're testing
• Stop marking system-test read-only in pods and instead stash any changes and then update
• Add a hook to remove any output in notebooks before commit or push

[rra]

Note that step four (not making it read-only) will mean that we'll need to do different things for system-test and tutorial-notebooks, which means we'll want to reconsider how this integration is done in the lab Docker image and how information is passed to it in environment variables by the lab controller and by Nublado v2.