refactor: Enhance docs, remove main, add tests in NodeStack

[Hardvan]

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized it.
• All filenames are in PascalCase.
• All functions and variable names follow Java naming conventions.
• All new algorithms have a URL in their comments that points to Wikipedia or other similar explanations.
• All new code is formatted with clang-format -i --style=file path/to/your/file.java

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.12%. Comparing base (1e50706) to head (fa6b883).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #6017      +/-   ##
============================================
+ Coverage     69.86%   70.12%   +0.25%     
- Complexity     4738     4748      +10     
============================================
  Files           633      633              
  Lines         17303    17279      -24     
  Branches       3326     3326              
============================================
+ Hits          12089    12117      +28     
+ Misses         4757     4704      -53     
- Partials        457      458       +1     

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