refactor: Enhance docs, add more tests in `Vigenere`

[Hardvan]

• [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 72.30%. Comparing base (e6f7063) to head (38088e0).

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #5899   +/-   ##
=========================================
  Coverage     72.29%   72.30%           
- Complexity     4957     4959    +2     
=========================================
  Files           650      650           
  Lines         17548    17552    +4     
  Branches       3384     3384           
=========================================
+ Hits          12687    12691    +4     
  Misses         4383     4383           
  Partials        478      478           

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