Add workflow overview image to CONTRIBUTING.md

[moylstmed360]

Fixes #1361: Adds an image of the pull-request steps and the workflow files that run at each step, and updates the Python/NumPy versions under "Review process".

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.08%. Comparing base (06563cf) to head (f434dc1).
⚠️ Report is 109 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1362      +/-   ##
==========================================
+ Coverage   88.88%   89.08%   +0.20%     
==========================================
  Files          56       57       +1     
  Lines        6439     6505      +66     
==========================================
+ Hits         5723     5795      +72     
+ Misses        716      710       -6     

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