Update paper branch
Summary
This MR introduces the following changes
- adds a "how to contribute" section to the documentation
- adds a tutorial based on the JetClass dataset
Relates to the following issues
- https://github.com/umami-hep/umami/issues/4
- https://github.com/umami-hep/umami/issues/2
- https://github.com/umami-hep/umami/issues/1