refactor: Improve readability and code clarity in InfixToPostfix

[alxkm]

• Added JavaDoc comments to the InfixToPostfix class and its methods to clearly explain their purpose, parameters, and behavior.
• Renamed local variables for better readability without changing the existing API.
• Added inline comments within methods to clarify the logic and improve code understandability.
• Enhanced exception message.

• 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

Attention: Patch coverage is 85.71429% with 3 lines in your changes missing coverage. Please review.

Project coverage is 74.63%. Comparing base (f325279) to head (704eb50).

Files with missing lines	Patch %	Lines
.../java/com/thealgorithms/stacks/InfixToPostfix.java	85.71%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #6362   +/-   ##
=========================================
  Coverage     74.63%   74.63%           
  Complexity     5452     5452           
=========================================
  Files           682      682           
  Lines         19017    19017           
  Branches       3674     3674           
=========================================
  Hits          14193    14193           
  Misses         4265     4265           
  Partials        559      559           

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