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.