Add documentation on how to use thebe(-light) for creating interactive coding exercises

[joelostblom]

In jupyterlite/jupyterlite#1333, I asked about using jupyter book / jupyter lite for creating a textbook with interactive coding exercises. It was suggested that this might be possible with thebe-light but that there are currently no docs on how to do it, so I'm opening this issues to suggest that this is added to the docs. See the issues linked above for a more exact description of what I have in mind.

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