Notebooks from other repos

[dglmoore]

Add some kind of "hey, there's a notebook in this other repo" functionality. That way we can write project-specific notebooks, keep them in the project's repo, but still host them on the site without duplicating the source.

Maybe something like:

1. Create a directory for the notebook in notebooks
2. Add a JSON file called repo.json which looks something like

   {
   "repo": "https://github.com/39alpha/project",
   "notebook": "notebooks/some-notebook/notebook.qmd",
   "version": <a git-ref for versioning>
   }