Improved documentation + umami paper draft
Summary
This MR introduces the following changes
- improves documentation by adding a tutorial designed for non-ATLAS users and contribution guidelines
- add paper documenting umami
Relates to the following issues
- https://github.com/openjournals/joss-reviews/issues/5833
- https://github.com/umami-hep/umami/issues/1
- https://github.com/umami-hep/umami/issues/2
- https://github.com/umami-hep/umami/issues/3
- https://github.com/umami-hep/umami/issues/4