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

[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

Attention: Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 70.61%. Comparing base (196cc60) to head (f6b2b3e).

Files with missing lines	Patch %	Lines
...n/java/com/thealgorithms/ciphers/ADFGVXCipher.java	75.00%	1 Missing :warning:

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #5897   +/-   ##
=========================================
  Coverage     70.61%   70.61%           
  Complexity     4839     4839           
=========================================
  Files           648      648           
  Lines         17507    17507           
  Branches       3374     3374           
=========================================
  Hits          12362    12362           
  Misses         4681     4681           
  Partials        464      464           

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