Add participating groups to README/docs

[sloede]

Supersedes #1880.

Note I also fixed a few discrepancies between the README and the docs.

[github-actions]

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[jmark]

I think the DLR also has a fair share in the development and should be mentioned as participating group:

[Dept. of High-Performance Computing, Institute of Software Technology, German Aerospace Center (DLR) :de:](https://www.dlr.de/en/sc/about-us/departments/high-performance-computing)

[torrilhon]

As discussed some time ago, I think, it should be my pull-request #1880 that is being merged, I already incorporated the suggestions of Michael.

[sloede]

Superseded by #2187