Update documentation to use sphinx-immaterial theme

[msimberg]

The current theme, sphinx-material, isn't very actively developed: bashtage/sphinx-material#141. The author even suggests using sphinx-immaterial: https://github.com/jbms/sphinx-immaterial. This PR updates the documentation to use sphinx-immaterial, but relies on jbms/sphinx-immaterial#338 at the moment for some newer features.

A few improvements:

• automatic dark mode (as well as manual override)
• interactive search
• (nicer) version-added roles (available in vanilla sphinx/rst, but the look better with sphinx-immaterial)
• generally slightly more polished style and responsive

I've also changed all the logos in the Acknowledgements to pngs, optimized them, and added a white background to the ETHZ logo (it was previously an svg with transparent background, leading to black text on black background in dark mode).

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
✅ -0.03% (target: -1.00%)	✅ ∅ (target: 90.00%)

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (8f08551)	18282	13806	75.52%
Head commit (18d8f87)	18282 (+0)	13801 (-5)	75.49% (-0.03%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#1311)	0	0	∅ (not applicable)

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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[msimberg]

To view the documentation locally, download the artifact created in the "Documentation" workflow (see workflow for a link), extract the archive, and serve it e.g. with python -m http.server --directory <extracted-dir>. You can also simply open index.html from the archive, but e.g. search won't work unless served with a webserver.

[msimberg]

Given that it's easy to use the unreleased version of sphinx-immaterial, and the official documentation only needs to build successfully in exactly one environment, I think it's relatively safe to rely on the unreleased version. I've pushed some final cleanups to this branch so it's ready to be reviewed.