docs(docs): add Approach diagrams and Explanation sections to LeetCode problem documentation

apps/docs/content/(tree)/104-maximum-depth-of-binary-tree.mdx

@@ -21,6 +21,16 @@ sidebar:
 - The number of nodes in the tree is in the range [0, 10^4].
 - `-100 <= Node.val <= 100`
 
+## Approach
+
+```mermaid
+flowchart TD
+  S(["maxDepth(root)"]) --> B{"root is None?"}
+  B -- yes --> Z(["return 0"])
+  B -- no --> L["l = maxDepth(root.left), r = maxDepth(root.right)"]
+  L --> E(["return 1 + max(l, r) — this node adds one level above its deeper subtree"])
+```
+
 ## Solution
 
 ```py
@@ -37,3 +47,7 @@ class Solution:
 
         return 1 + max(self.maxDepth(root.left), self.maxDepth(root.right))
 ```
+
+## Explanation
+
+<YouTube id="hTM3phVI6YQ" title="Maximum Depth of Binary Tree - 3 Solutions - Leetcode 104 - Python" />