Prepare version 0.19.1

[Zeitsperre]

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
  • This PR fixes #xyz
• (If applicable) Documentation has been added / updated (for bug fixes / features).
• (If applicable) Tests have been added.
• CHANGELOG.rst has been updated (with summary of main changes).
  • Link to issue (:issue:number) and pull request (:pull:number) has been added.

What kind of change does this PR introduce?

• Prepares the next patch version (v0.19.1)

Does this PR introduce a breaking change?

For users, no; For developers, yes:

• The default branch is now main
• The development tooling has been streamlined to use ruff primarily
• The code base has been significantly modified to adhere to stricter standards
• The registry.txt entries now also support the notebooks

[tlvu]

LGTM