Add learn notebook on the CLs method

[matthewfeickert]

Summary

Based off of emails and conversations related to:

• hypotest documentation not clear on return types depending on test statistic chosen #2262
• Clarify in docstrings what an expected CLs value is #1620
• doc: Add page describing how to build Upper Limits table pyhf/pyhf-tutorial#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

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