Clean up some unused notebooks

There are some notebooks still in the repository, that aren't actually linked to from anywhere in the site. They can be reached with the right url, but they're not considered "live" on the site.

This is not a critical issue, because there's nothing wrong or sensitive in the notebooks. But they should be either completed, moved to a development branch, or deleted. What do with these notebooks should be more clear as the project evolves over the next few months.