fix: poor implementation of avl tree

[bertrand-awz]

Description of Change

The previous AVL implementation did not rebalance the tree after deletion and managed nodes through raw pointers. This change replaces it with a reusable C++17 AvlTree<T> class that rebalances after insertion and deletion and caches node heights.

• Uses std::unique_ptr for automatic memory management.
• Supports initializer-list construction, copying, moving, searching, and clearing.
• Provides size, height, and emptiness queries; duplicate values are ignored.
• Adds terminal visualization with the root above its children and diagonal Unicode arrows, without external dependencies.
• Moves the implementation into data_structures/avl_tree/ and updates CMake, DIRECTORY.md, documentation, and the usage example.

Resolve issue mentionned in ToDo List of this project. Here is the link: https://thealgorithms.github.io/C-Plus-Plus/d8/dee/avltree_8cpp.html

Checklist

• Added description of change
• Added file name matches File name guidelines
• Added tests and example, test must pass
• Added documentation so that the program is self-explanatory and educational - Doxygen guidelines
• Relevant documentation/comments is changed or added
• PR title follows semantic commit guidelines
• Search previous suggestions before making a new one, as yours may be a duplicate.
• I acknowledge that all my contributions will be made under the project's license.