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 74.65%. Comparing base (0a4f554) to head (759d5da).

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     74.64%   74.65%   +0.01%     
  Complexity     5450     5450              
============================================
  Files           682      682              
  Lines         19013    19013              
  Branches       3672     3672              
============================================
+ Hits          14193    14195       +2     
+ Misses         4262     4261       -1     
+ Partials        558      557       -1     

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