ci: add check formatting workflow

[VJanKraemer]

Subject

Add a workflow to check for correct clang-format source code formatting!

Developer checklist (address before review)

• [ ] Changelog.md updated
• [ ] Prepared update for depending repositories
• [ ] Documentation updated (public API changes only)
• [ ] API docstrings updated (public API changes only)
• [ ] Rebase → commit history clean
• [ ] Squash and merge → proper PR title