documentation: Create CONTRIBUTING.md

[sueszli]

Subset of: #5597

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.16%. Comparing base (b230f09) to head (55dc464).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5604   +/-   ##
=======================================
  Coverage   86.16%   86.16%           
=======================================
  Files         394      394           
  Lines       56390    56390           
  Branches     6496     6496           
=======================================
  Hits        48588    48588           
  Misses       6269     6269           
  Partials     1533     1533           

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