Add UI tests

[jtpio]

Fixes #70

Closes #71
Closes #69

• Use pixi to set up the environment
• Add UI tests to check all the notebooks

[github-actions]

👈 Try it on ReadTheDocs

[jtpio]

Performed a couple of restarts to check for flakiness, and looking good so far:

[martinRenou]

Thanks a lot @jtpio !