doc: add example and doctest for gamma

[inkydragon]

I'm not sure if those examples are too long, and if they are, I can break them up into the .md docs.

Compare to ...

• python - SciPy: https://docs.scipy.org/doc/scipy/reference/generated/scipy.special.gamma.html
• Matlab: https://www.mathworks.com/help/matlab/ref/gamma.html

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HTML output:

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (8072ed6) 94.40% compared to head (5344d64) 94.40%.
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #462   +/-   ##
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  Coverage   94.40%   94.40%           
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  Files          14       14           
  Lines        2914     2914           
=======================================
  Hits         2751     2751           
  Misses        163      163           

Flag	Coverage Δ
unittests	94.40% <ø> (ø)

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[ViralBShah]

I think long is ok. This isn't that long. Please merge if ready to go.