Improve handling of packages used for the demo deployed on GitHub Pages

[jtpio]

Currently the dependencies used for the demo are simply installed in the workflow directly:

ai/.github/workflows/deploy.yml

Lines 21 to 26 in a3fda1c

	- name: Install the dependencies
	run: |
	python -m pip install jupyterlite-pyodide-kernel jupyterlite-core --pre
	# install a dev version of the extension
	python -m pip install .

But this will become more difficult to track if we want to add more dependencies in the future.

So opening an issue to look into this at some point.

[brichet]

We could add a demo_requirements.txt at the root of the project.
It will also be used when we'll provide a documentation, if we want to include the deployment there.

[jtpio]

Another option could be to use an environment.yml file with jupyterlite-xeus.

[jtpio]

uv could also work well, especially with its lock file that would avoid having dependencies drift away.