Add a static demo with per-PR review

[bollwyvl]

Problem

There's no place to try out this kernel yet.

Aside from off main, as new features are added, it would be nice to have something that reviewers can try out.

Suggested Improvement

• add a dev-focused binder with jupyter-server-proxy
• get set up for readthedocs (doesn't even require the mamb flag anymore to get a self-consistent environment)

[jtpio]

Actually there is one in the https://github.com/jupyterlite/demo repo: https://github.com/jupyterlite/demo/blob/main/content/xeus-sqlite/simple-operations.ipynb

Alongside other xeus-based kernels.

[bollwyvl]

Right, that's a starting point, but stuff has to hit main to show up there, presumably...

[psychemedia]

I also note that this repo builds on jupyter-xeus/xeus-sqlite, and that repo has upstream fixes such as jupyter-xeus/xeus-sqlite#119 that will need to be tracked here and in downstream demo environments if folk want to demo latest.

[jtpio]

With the ongoing refactoring of the xeus-based lite kernels, it's also possible the lite assets will eventually be developed and shipped from the main jupyter-xeus/xeus-sqlite repo directly.

Making https://github.com/jupyterlite/xeus-sqlite-kernel not strictly necessary.