DRAFT: Add ability to generate a notebook index for Try QuTiP.

[hodgestar]

This PR updates the create_index.py script to allow generating a Markdown notebook index for Try QuTiP.

It also cleans up many small details of the script.

I was hoping to make the notebook generation more general, but JupyterLite doesn't seem to handle relative URLs particularly well so I ended up needing to hardcode the Try QuTiP website URL in a similar way to how we do for NBViewer. If we find a way to open notebooks from relative links in other notebooks, we remove the hardcoded URL and formatting and make this more general.

We could also link directly to Try QuTiP from the notebook page, which might be a simpler approach.