fix(docs): Improve the documentation for the new release

[sylvaincom]

Fixing parts of https://github.com/probabl-ai/skore/issues/627

• removed getting started and user guide tabs as they are in examples
• added a convincing getting started example following @koaning advice (thanks again)
• merged the two examples on the skore UI into one
• In CONTRIBUTING.md, made a link to the documentation
• Updated the GIF in the README and in the documentation
• In the cross-validation example, no longer use Auguste's screenshot but in the notebook I export a png that is then loaded
  • added kaleido to pyproject.toml