Add "Used by"

[utensil]

Add information on how GAlgebra is used in other projects.

It's worth it to keep it inside the repo, and not as a hanging issue.

This list provides information on GAlgebra's usefulness for some, and these downstream projects can be used to validate the GAlgebra against them.

This list has not yet include results of tracing citations, which is also worth gathering. (done)

I'll leave the PR open a bit for feedback.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.96%. Comparing base (327df0a) to head (542f137).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #528   +/-   ##
=======================================
  Coverage   78.96%   78.96%           
=======================================
  Files          19       19           
  Lines        4402     4402           
=======================================
  Hits         3476     3476           
  Misses        926      926           

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[utensil]

Citation tracing (in recent years, non-exhaustive) is done by:

1. https://scholar.google.com/scholar?oi=bibs&hl=en&cites=6283120175695000074&as_sdt=5
2. citations on https://zenodo.org/records/11097345