[contributors wanted] Documentation and tutorial overhaul

[jonas-eschle]

The documentation is grown over time, so did the tutorials.

Different tutorials:

• PyHEP talks: https://github.com/zfit/PyHEP2022, https://github.com/zfit/zfit-PyHEP
• entry docs in zfit, these are simple pieces of doc: https://zfit.readthedocs.io/en/latest/tutorials/introduction.html
• the zfit tutorials that contain different notebooks
• Sebastians starterkit talk: https://github.com/schmitse/lhcb-starterkit-2022
• analysis-essentials

Some of these overlap, most have at least parts outdated with old syntax. The goal is to go through them (or parts) to upgrade and re-organize.

The reorganization should include a few new tutorials, either smaller ones on specific topics or larger ones, demonstating a workflow.

Tests generate images automatically, showcasing many parts of zfit (distributions etc) into docs/images/_generated_by_tests. They can be incorporated into the documentation

Please get in contact if you are interested to do parts of it (just comment here) or have tutorials that you want to contribute