refactor: StackPostfixNotation

[alxkm]

Change Map.ofEntries test to Parametrized tests.
Documentation changes.

• 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 48.02%. Comparing base (b70f077) to head (ca8ead0).

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #5400   +/-   ##
=========================================
  Coverage     48.02%   48.02%           
- Complexity     2962     2963    +1     
=========================================
  Files           524      524           
  Lines         15275    15275           
  Branches       2903     2903           
=========================================
  Hits           7336     7336           
  Misses         7637     7637           
  Partials        302      302           

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