docs: update contributing guide

[jh-leong]

Update the repository link and recommend using the official Pinia playground for bug reports instead of jsfiddle.net :)

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commit: 1eb7c33

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.41%. Comparing base (0d8bce8) to head (1eb7c33).
Report is 1 commits behind head on v2.

Additional details and impacted files

@@            Coverage Diff             @@
##               v2    #2782      +/-   ##
==========================================
- Coverage   92.43%   90.41%   -2.03%     
==========================================
  Files          14       14              
  Lines        1335     1335              
  Branches      220      220              
==========================================
- Hits         1234     1207      -27     
- Misses        101      127      +26     
- Partials        0        1       +1     

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[posva]

Thanks!