Added AVL tree implementation with detailed comments

Author: amarmaurya-com

src/test/java/com/thealgorithms/tree/AVLTest.java

@@ -95,7 +95,7 @@ private Node insertRecursive(Node node, int key) {
             node.right = insertRecursive(node.right, key);
         } else {
             return node; // duplicates ignored
-            }
+        }
 
         updateHeight(node);
         return balanceNode(node);