Add JupyterLite to our documentation

[choldgraf]

Context

Many of the communities in the PyData ecosystem have started to use JupyterLite in order to provide rapid interaction with their documentation (for the workflows that are possible with JupyterLite and WASM, anyway).

For example, here's Pandas using it: https://pandas.pydata.org/getting_started.html

I suspect many more projects in the ecosystem might benefit from it as well, and would appreciate if they had some inspiration and guidance for how to enable it in Sphinx.

Proposal

Given that many PyData projects use or build on this theme, we should use JupyterLite in our own documentation, both in order to dog-food it and become more experienced with it, and as a way to demonstrate how it works and how others can set it up with the theme.