Improve documentation for CountNiceSubarrays Sliding Window Solution

[shalinijayakumar16]

• Added detailed JavaDoc explaining the approach
• Included example input/output
• Improved code readability with comments
• No logic changes

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.08%. Comparing base (ba5ccbe) to head (18bb801).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #7224      +/-   ##
============================================
- Coverage     79.09%   79.08%   -0.01%     
+ Complexity     6973     6972       -1     
============================================
  Files           783      783              
  Lines         22950    22950              
  Branches       4512     4512              
============================================
- Hits          18152    18151       -1     
  Misses         4077     4077              
- Partials        721      722       +1     

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