Various documentation and CI updates

[MakisH]

• Bump default OpenFOAM version from v2306 to v2312.
• Update badges, add a link to the forum in the badges, to direct people there for questions, instead of the issues.
• Build CI workflow: switch from the setup-precice-action (slow) to precice/precice:nightly.
• Move shellcheck to pre-commit and rename workflow.

TODO list:

• I updated the documentation in docs/
• I added a changelog entry in changelog-entries/ (create directory if missing) -> N/A

[MakisH]

I also switched from using the preCICE Debian packages to the setup-precice-action in the build-custom: https://github.com/precice/openfoam-adapter/actions/runs/8364095082

[MakisH]

Something seems to be completely broken in regarding the step outputs in GitHub Actions. Even though we don't seem to have changed anything related, the value of openfoam_exec we set in the InstallOpenFOAM step does not propagate to the adapter installation step.

Documentation: https://docs.github.com/en/actions/using-workflows/workflow-commands-for-github-actions#setting-an-output-parameter

I suspect that there is either a bug with GitHub Actions, or something changed and is not documented.

Example job that fails: https://github.com/precice/openfoam-adapter/actions/runs/8365204318/job/22902472048

Note that I could install OpenFOAM v2206 on Ubuntu 22.04, but several other combinations seemed to lead to various sourceforge-related issues (404 for various subpackages).

[MakisH]

The workflow is broken even on develop. This PR has nothing to do with it. I give up and merge.