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

[Hardvan]

…edListRecursion`

• [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 70.18%. Comparing base (e1a0155) to head (19332fa).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #6012      +/-   ##
============================================
+ Coverage     70.12%   70.18%   +0.06%     
- Complexity     4748     4755       +7     
============================================
  Files           633      633              
  Lines         17279    17272       -7     
  Branches       3326     3322       -4     
============================================
+ Hits          12117    12123       +6     
+ Misses         4704     4693      -11     
+ Partials        458      456       -2     

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.