refactor: Enhance docs, add more tests in `NonRepeatingNumberFinder`

[Hardvan]

• [x] I have read CONTRIBUTING.md.
• [x] This pull request is all my own work -- I have not plagiarized it.
• [x] All filenames are in PascalCase.
• [x] All functions and variable names follow Java naming conventions.
• [x] All new algorithms have a URL in their comments that points to Wikipedia or other similar explanations.
• [x] All new code is formatted with clang-format -i --style=file path/to/your/file.java

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 70.16%. Comparing base (82dee61) to head (6f7a0d9).

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #5843   +/-   ##
=========================================
  Coverage     70.16%   70.16%           
  Complexity     4754     4754           
=========================================
  Files           634      634           
  Lines         17295    17295           
  Branches       3327     3327           
=========================================
  Hits          12135    12135           
  Misses         4704     4704           
  Partials        456      456           

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