Evaluate use with jupyterlite, add demo to docs

[Nicholas Bollweg]

As of jupyterlite 0.1.0b10, emscripten-based kernels like pyolite and xeus-python have direct access to the /files deployed with the site as well as those created in-browser or added via drag-drop.

This is very nice for enabling the importnb two-notebook approach: one for show that imports and uses the one for pros, which has more details and tests.

With pyolite, it's reasonably straightforward to use --piplite-wheels pointed at an as-built wheel, which means per-PR-push ReadTheDocs previews can contain the actual development code, and github pages can be made to work as well.