GIT: added badges for documentation, tests and pre-commit

[rpartzsch]

Refactored standard tests into pre-commit and pytests.
Also added Github badges for the three actions (tests, pre-commit and documentation).

[cbespin]

Does the pre-commit workflow just run the hook again? Or check if the hook was run (which would be handled by git locally, I assume)? I am not sure I understand the reason but I have also never used it

[rpartzsch]

The way I understand it is, pre-commit runs the hooks defined in .pre-commit-config.yaml again. Changes are not commited, but error codes are raised if changes would be needed. So if black fails in development then the pre-commit badge would also show failing.