docs: document main

Co-authored-by: David Leal <halfpacho@gmail.com>

data_structures/avltree.cpp

@@ -176,6 +176,10 @@ void levelOrder(node *root) {
     }
 }
 
+/**
+ * @brief Main function
+ * @returns 0 on exit
+ */
 int main() {
     // Testing AVL Tree
     node *root = nullptr;