Add learn notebook on the CLs method

[matthewfeickert]

trafficstars

Summary

Based off of emails and conversations related to:

• https://github.com/scikit-hep/pyhf/issues/2262
• https://github.com/scikit-hep/pyhf/issues/1620
• https://github.com/pyhf/pyhf-tutorial/pull/57

and others it would be good to have a "learn" style notebook that builds up the motivation and the geometric intuition for the CLs method as a ratio of probability.

It would probably make sense to have this conceptual notebook, that might not use pyhf much at all, be either under "Fundamentals" or be under a new section called something along the lines of "Concepts".

Documentation Page Link

https://pyhf.readthedocs.io/en/v0.7.2/learn.html

Code of Conduct

• [X] I agree to follow the Code of Conduct