docs: add github actions example for private repo

[martijnhalekor]

A minor contribution to the docs. My very first contribution to open source, so let me know if I'm doing this right 🤗

As requested in unjs/community#43

[pi0]

Thanks ❤️

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 43.82%. Comparing base (743c9ae) to head (fc8643a).
Report is 36 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #197       +/-   ##
===========================================
- Coverage   54.63%   43.82%   -10.81%     
===========================================
  Files           7        6        -1     
  Lines         615      470      -145     
  Branches       47       47               
===========================================
- Hits          336      206      -130     
+ Misses        278      263       -15     
  Partials        1        1               

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