refactor: Enhance docs, code, add tests in KeithNumber

[Hardvan]

• 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

❌ Patch coverage is 61.11111% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.32%. Comparing base (297634d) to head (50b2174).

Files with missing lines	Patch %	Lines
...main/java/com/thealgorithms/maths/KeithNumber.java	61.11%	7 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #6748      +/-   ##
============================================
+ Coverage     77.23%   77.32%   +0.09%     
- Complexity     6145     6151       +6     
============================================
  Files           720      720              
  Lines         20533    20536       +3     
  Branches       3974     3974              
============================================
+ Hits          15858    15879      +21     
+ Misses         4068     4051      -17     
+ Partials        607      606       -1     

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[alxkm]

Looks good. Thank you for the contribution.