DigraphDegeneracy improvements

[mtorpey]

I went on a personal journey to understand digraph degeneracy, which took me longer than it should have.

This PR:

• improves the doc to make it simpler and clearer;
• adds a test that checks the advertised property of degeneracy ordering

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.38%. Comparing base (cb11a3f) to head (dc5497a).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #921      +/-   ##
==========================================
- Coverage   97.41%   97.38%   -0.03%     
==========================================
  Files          50       50              
  Lines       21175    21193      +18     
  Branches      639      639              
==========================================
+ Hits        20627    20639      +12     
- Misses        483      489       +6     
  Partials       65       65              

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