Migrate to standard Pip installation

The dependency on Poetry regularly leads to installation issues for students. And technically, everything we use Poetry for can now also be done with a standard Pip install and a standard-compliant pyproject.toml configuration. I've refactored the pyproject.toml file, installation and update commands to not rely on Poetry anymore.

The only caveat for now is that the webis command once needs to be re-installed by running the ./install.sh script. If needed, I could also prepare a "smoother" transition that auto-migrates from Poetry to Pip.