add ergonomic CLI

This adds the command check SUBMISSION_NUMBER DIRECTORY to do the equivalent of what

pytest submission_checker --path $DIRECTORY --tb=line -m submission$SUBMISSION_NUMBER

would do before.

(The poetry install step and the poetry run prefix for the check command will be gone when this software can be installed as a Python package.)

I've removed the documentation on how to run individual "rules" (individual checks) instead of a whole "ruleset" (all checks for a specific Testat-Abgabe), because

• the new CLI with the check command doesn't offer this for now
• the target audience (students and teaching assistants) shouldn't anymore be required to care about pytest for using this software
• the target audience (students and teaching assistants) probably won't need that degree of granularity in the invocation
  • If we later notice that there is a need, we can add it as a new command or as an option to check.