Add time and space complexity documentation to LongestNonRepetitiveSubstring

[eMohan07]

This pull request adds detailed time and space complexity documentation to the method
lengthOfLongestSubstring in LongestNonRepetitiveSubstring.java.

The update improves clarity by explaining the efficiency of the sliding window
approach used in the implementation. No functional changes have been made.

Type of Change

• Documentation update

Checklist

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized it.
• No functional changes introduced.
• Code compiles successfully.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 88.18227% with 236 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.34%. Comparing base (29ce2ef) to head (2befeec).

Files with missing lines	Patch %	Lines
.../java/com/thealgorithms/ciphers/ProductCipher.java	0.00%	44 Missing ⚠️
.../com/thealgorithms/backtracking/MazeRecursion.java	55.55%	12 Missing and 4 partials ⚠️
.../java/com/thealgorithms/ciphers/ElGamalCipher.java	57.89%	6 Missing and 10 partials ⚠️
...java/com/thealgorithms/ciphers/PlayfairCipher.java	80.48%	12 Missing and 4 partials ⚠️
...lgorithms/ciphers/ColumnarTranspositionCipher.java	78.12%	11 Missing and 3 partials ⚠️
...va/com/thealgorithms/backtracking/KnightsTour.java	75.55%	5 Missing and 6 partials ⚠️
...a/com/thealgorithms/backtracking/SudokuSolver.java	73.80%	10 Missing and 1 partial ⚠️
...n/java/com/thealgorithms/backtracking/NQueens.java	68.75%	10 Missing ⚠️
.../java/com/thealgorithms/ciphers/AESEncryption.java	66.66%	9 Missing ⚠️
src/main/java/com/thealgorithms/ciphers/DES.java	92.74%	6 Missing and 3 partials ⚠️
... and 29 more

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #7283   +/-   ##
=========================================
  Coverage     79.34%   79.34%           
  Complexity     7006     7006           
=========================================
  Files           784      784           
  Lines         22992    22992           
  Branches       4516     4516           
=========================================
  Hits          18242    18242           
  Misses         4020     4020           
  Partials        730      730           

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[vil02]

There is something clearly wrong with this change - the diff does not match the description.