refactor: Enhance docs, add tests in `AVLSimple`

[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 40.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 72.58%. Comparing base (bca8d0e) to head (33a4cc9).

Files with missing lines	Patch %	Lines
.../thealgorithms/datastructures/trees/AVLSimple.java	40.00%	2 Missing and 1 partial :warning:

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #6057      +/-   ##
============================================
+ Coverage     72.40%   72.58%   +0.18%     
- Complexity     4983     4996      +13     
============================================
  Files           653      653              
  Lines         17574    17577       +3     
  Branches       3386     3386              
============================================
+ Hits          12724    12758      +34     
+ Misses         4371     4332      -39     
- Partials        479      487       +8     

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