refactor: Enhance docs, add tests in AVLTree

[Hardvan]

• 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

Attention: Patch coverage is 85.18519% with 4 lines in your changes missing coverage. Please review.

Project coverage is 72.97%. Comparing base (e94be71) to head (0c90a96).

Files with missing lines	Patch %	Lines
...om/thealgorithms/datastructures/trees/AVLTree.java	85.18%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@ Coverage Diff @@ ## master #6058 +/- ## ============================================ + Coverage 72.42% 72.97% +0.54%  - Complexity 4991 5027 +36  ============================================ Files 655 655 Lines 17593 17588 -5 Branches 3388 3387 -1 ============================================ + Hits 12742 12834 +92  + Misses 4371 4261 -110  - Partials 480 493 +13 

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