refactor: Enhance docs, remove main, add tests in `CountSinglyLinke…

[Hardvan]

…dListRecursion`

• 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

All modified and coverable lines are covered by tests ✅

Project coverage is 68.76%. Comparing base (ed35374) to head (1398b68).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5992      +/-   ##
============================================
+ Coverage     68.72%   68.76%   +0.03%     
- Complexity     4630     4634       +4     
============================================
  Files           620      620              
  Lines         17112    17107       -5     
  Branches       3302     3300       -2     
============================================
+ Hits          11761    11764       +3     
+ Misses         4900     4892       -8     
  Partials        451      451              

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