chore: clean up content phase organization

apps/docs/content/(03-hierarchical)/tree-dfs/104-maximum-depth-of-binary-tree.mdx

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+---
+title: '104. Maximum Depth of Binary Tree'
+description: Given the root of a binary tree, return its maximum depth
+sidebar:
+  label: 'Maximum Depth of Binary Tree'
+  badge: 'Easy'
+---
+
+<Badge variant="accent">Tree</Badge>
+
+### Example 1:
+- Input: `root = [3,9,20,null,null,15,7]`
+- Output: `3`
+
+### Example 2:
+- Input: `root = [1,null,2]`
+- Output: `2`
+
+### Constraints:
+
+- 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
+# Definition for a binary tree node.
+# class TreeNode:
+#     def __init__(self, val=0, left=None, right=None):
+#         self.val = val
+#         self.left = left
+#         self.right = right
+class Solution:
+    def maxDepth(self, root: Optional[TreeNode]) -> int:
+        if not root:
+            return 0
+
+        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" />