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

[Hardvan]

…yLinkedList`

• 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 68.11%. Comparing base (6f489e5) to head (147eeb7).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5997      +/-   ##
============================================
+ Coverage     67.97%   68.11%   +0.13%     
- Complexity     4587     4594       +7     
============================================
  Files           620      620              
  Lines         17114    17104      -10     
  Branches       3305     3300       -5     
============================================
+ Hits          11634    11650      +16     
+ Misses         5035     5009      -26     
  Partials        445      445              

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