Follow-up for after the JB2 switch

Here are items we left for follow-ups after #274

• build time on CircleCI is significantly longer than it was with JB1. I could easily consider this as a follow-up though.

From rossbar: I'm considering this "resolved" - I'm fairly confident the issue was the tox configuration not passing in the OMP_NUM_THREADS variable to the build environment. I just rebased #279 on main (after the JB2 switch) and re-ran the doc build. The sphinx-based run took 1m43 seconds to build, while JB2 took 1m17s 🚀

• cleaning up build errors and warnings -- currently we don't run with strict (and we didn't run with a similar manner as we were not failing JB1 builds with these broken links either)
• deployment not to a gh-pages branch -- this could probably be done as a follow-up, too and now just use the file we have. (I need to make my other repos consistent, too so can do this repo along with those).
• cleanup environment.yaml, switch to a more modern and reliable solution (eyeing pixi)
• change internal links to be myst syntax so they will have hover overs
  - [ ] Request extended amsmath support for LaTeX math rendering. See e.g. numpy.org/numpy-tutorials/mooreslaw-tutorial#building-moores-law-as-an-exponential-function. cf. jupyter-book/mystmd#2447 this has been worked around, but better LaTeX support would be nicer, which is an upstream issue
• Update README/contributor guide for new JB2 patterns.
• deploy GHA should not run on forks (in fact the CI should not get triggered to run on forks -- add in conditionals)

Prüfe #274 und die verbleibenden nicht abgehakten Punkte, beginnend mit der tox-Konfiguration, environment.yaml, dem README/contributor guide und dem Deployment-GitHub-Actions-Workflow. Bestätige, welche Build-Warnungen, Link-Syntax, welches Deployment-Verhalten, welche Umgebungs-Tools und welche Bedingungen für durch Forks ausgelöste Ausführungen weiterhin gelten. Erledigt bedeutet, dass die zutreffenden Folgearbeiten abgeschlossen oder in fokussierte Issues aufgeteilt wurden.