refactor: Enhance docs, add tests in AVLTree

[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 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 :warning:

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     

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