docs: Add CITATION.cff Citation File Format file

[matthewfeickert]

• Add Citation File Format file to repo to get repository cite button on GitHub.
  • c.f. https://docs.github.com/en/github/creating-cloning-and-archiving-repositories/creating-a-repository-on-github/about-citation-files
• Add CITATION.cff to tbump control.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.17%. Comparing base (1683d9c) to head (8a331b1).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #248   +/-   ##
=======================================
  Coverage   91.17%   91.17%           
=======================================
  Files           2        2           
  Lines         306      306           
  Branches       83       83           
=======================================
  Hits          279      279           
  Misses         17       17           
  Partials       10       10           

Flag	Coverage Δ
unittests-3.10	90.84% <ø> (ø)
unittests-3.11	90.84% <ø> (ø)
unittests-3.12	90.84% <ø> (ø)
unittests-3.8	90.84% <ø> (ø)
unittests-3.9	90.84% <ø> (ø)

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