Replace unsplash images with picsum

[stefanv]

@agoose77 I replaced the image links with ones that will work. I did not how how to generate myst_tests.ipynb from myst_tests.md (or vice versa), and I also cannot see where that is used. So, leaving this here as a step in the right direction, hopefully a quick follow-up from you to get it up and running.

[github-actions[bot]]

:point_left: Launch a Binder on branch stefanv/jupyterlab-myst/fix-unsplash-links

[stefanv]

I can't get the package built locally, FWIW.

[agoose77]

<3