docs: Add learn notebook on using calculators

[kratsg]

Description

This is the last thing from #709 that we don't have in pyhf by now.

Render: https://pyhf.readthedocs.io/en/docs-usingcalculators/examples/notebooks/learn/UsingCalculators.html

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• Tests are passing
• "WIP" removed from the title of the pull request
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* Add learn notebook on using calculators (asymptotics, toy-based)
   - Add notebook to notebook tests
* Update Sphinx Makefile to correctly raise warnings as errors
   - c.f. https://www.sphinx-doc.org/en/master/man/sphinx-build.html
   - Amends PR #1257

Co-authored-by: Lukas Heinrich <[email protected]>

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Codecov Report

Merging #1258 (f0b37ba) into master (316ff79) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1258   +/-   ##
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  Coverage   97.47%   97.47%           
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  Files          63       63           
  Lines        3716     3716           
  Branches      525      525           
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  Hits         3622     3622           
  Misses         55       55           
  Partials       39       39           

Flag	Coverage Δ
unittests	97.47% <ø> (ø)

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[matthewfeickert]

Thanks @kratsg. Just a quick suggestion from me and then I'll review later today when I'm finished with current work.

[matthewfeickert]

@kratsg I made some small formatting changes (tried to make it easy to revert any that you didn't like) and added this notebook to the notebook tests, but all LGTM! 👍