Docs: migrate Coveralls badge to Codecov

[ohmyfelix]

Closes contributte/contributte#72.

What changed

• replaced the README coverage badge link from Coveralls to Codecov
• confirmed the repository already uses the shared Codecov-ready coverage workflow, so no workflow changes were needed
• confirmed there is no obsolete tests/.coveralls.yml file to remove

Why

• finishes the remaining Coveralls-to-Codecov migration for this repository so coverage links stay consistent with the current reporting setup

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.50%. Comparing base (b90e7fd) to head (f5b9fb5).
⚠️ Report is 7 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #29      +/-   ##
==========================================
- Coverage   78.04%   77.50%   -0.55%     
==========================================
  Files           2        2              
  Lines         123      120       -3     
==========================================
- Hits           96       93       -3     
  Misses         27       27              

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