docs: enhance and refactor package examples

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Codecov Report

❌ Patch coverage is 0% with 109 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.70%. Comparing base (18f0d8a) to head (4d1f011).

Files with missing lines	Patch %	Lines
examples/future/main.go	0.00%	69 Missing ⚠️
examples/cyclic_barrier/main.go	0.00%	32 Missing ⚠️
examples/goroutine/main.go	0.00%	8 Missing ⚠️

❌ Your patch status has failed because the patch coverage (0.00%) is below the target coverage (70.00%). You can increase the patch coverage or adjust the target coverage.
❌ Your project status has failed because the head coverage (77.70%) is below the target coverage (80.00%). You can increase the head coverage or adjust the target coverage.
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Additional details and impacted files

@@             Coverage Diff             @@
##           master      #45       +/-   ##
===========================================
- Coverage   88.02%   77.70%   -10.33%     
===========================================
  Files          18       19        +1     
  Lines         685      776       +91     
===========================================
  Hits          603      603               
- Misses         70      161       +91     
  Partials       12       12               

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