docs(docs): add ProblemBadges component, layout slot, and enrich pages with topics/video frontmatter, plus guide placeholders and topic‑landing map.

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

@@ -2,13 +2,17 @@
 title: '104. Maximum Depth of Binary Tree'
 description: Given the root of a binary tree, return its maximum depth
 icon: dot
+topics:
+  - { name: "Tree", slug: "tree" }
+  - { name: "Depth-First Search", slug: "depth-first-search" }
+  - { name: "Breadth-First Search", slug: "breadth-first-search" }
+  - { name: "Binary Tree", slug: "binary-tree" }
+video: "hTM3phVI6YQ"
 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`
@@ -48,7 +52,3 @@ 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" />