docs: add JavaDoc to AreaOfPolygon algorithm

[dyrpsf]

This PR adds JavaDoc comments to the AreaOfPolygon class and its method.
These explain the Shoelace formula for calculating the area of a polygon based on its coordinates.

🛠️ Changes Made

• JavaDoc added above class and method in AreaOfPolygon.java
• Sample usage included to demonstrate output
• No logic changed — documentation only

🧩 Issue

Fixes #6672

✅ Checklist

• 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

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.77%. Comparing base (6b7d201) to head (5201d20).

Files with missing lines	Patch %	Lines
...in/java/com/thealgorithms/maths/AreaOfPolygon.java	0.00%	12 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #6770      +/-   ##
============================================
- Coverage     77.82%   77.77%   -0.05%     
+ Complexity     6392     6391       -1     
============================================
  Files           732      733       +1     
  Lines         21320    21332      +12     
  Branches       4182     4184       +2     
============================================
- Hits          16592    16591       -1     
- Misses         4059     4071      +12     
- Partials        669      670       +1     

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[github-actions]

This pull request has been automatically closed because its workflows or checks failed and it has been inactive for more than 14 days. Please fix the workflows and reopen if you'd like to continue. Merging from main/master alone does not count as activity.