Rework release workflow

[sieben-gea]

The current automated release workflow has some problems that I would like to discuss:

1. The workflow does not work as intended: It will release old commits which were never included in the current push.
2. It discourages pushing fixes to the main branch in the worry that they are not yet enough for a whole new release.
3. Automated releases increase the attack surface/opportunities for supply-chain attacks.
4. The workflow is doing things which are unexpected for new contributors (such as releasing code directly), which is an anti-pattern for long-term software sustainability.

I would like to avoid these problems. However, I am in favor of creating a release workflow which is triggered by a human publishing a release on GitHub, which then also builds the package and pushes it to PyPI.

[sieben-gea]

The workflow should also push releases to Zenodo.