Add Zenodo concept DOI

[galjos]

Summary

• Add the Zenodo concept DOI badge to the README.
• Add the concept DOI to citation metadata.

Validation

• DOI and badge resolve successfully.
• CFF 1.2 schema validation passed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.40%. Comparing base (5212523) to head (52ccde5).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #127   +/-   ##
=======================================
  Coverage   97.40%   97.40%           
=======================================
  Files          32       32           
  Lines        2001     2001           
=======================================
  Hits         1949     1949           
  Misses         52       52           

Flag	Coverage Δ
unittests	97.40% <ø> (ø)

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