Add contents.ipynb as start page

[dwhswenson]

This is an attempt to add workspaces for each of the "subtutorials" (beginner, intermediate, advanced) within the Binder JupyterLab experience.

In the end, I'm just going to add the contents.ipynb and try to make that open by default. More advanced single-click approaches to switch tutorial context would be nice for the future, but aren't happening today.

[dwhswenson]

Possibly relevant information to pick this up in the future:

• https://github.com/ian-r-rose/binder-workspace-demo/
• https://jupyterlab.readthedocs.io/en/stable/user/urls.html
• Example of tweaking a template workspace here: https://github.com/dask/dask-examples/blob/dcf2ca5ef715c1638ae5fdcefc3f4b1c0b4cc703/binder/start#L4-L10