docs(docs): add page icon frontmatter to problem pages

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

@@ -1,6 +1,7 @@
 ---
 title: '104. Maximum Depth of Binary Tree'
 description: Given the root of a binary tree, return its maximum depth
+icon: line-dot-left-horizontal
 sidebar:
   label: 'Maximum Depth of Binary Tree'
   badge: 'Easy'

apps/docs/content/(03-hierarchical)/tree-dfs/226-invert-binary-tree.mdx

@@ -1,6 +1,7 @@
 ---
 title: '226. Invert Binary Tree'
 description: Given the root of a binary tree, invert the tree, and return its root
+icon: line-dot-left-horizontal
 sidebar:
   label: 'Invert Binary Tree'
   badge: 'Easy'

apps/docs/content/(03-hierarchical)/tree-dfs/236-lowest-common-ancestor-of-a-binary-tree.mdx

@@ -1,6 +1,7 @@
 ---
 title: '236. Lowest Common Ancestor of a Binary Tree'
 description: Given a binary tree, find the lowest common ancestor (LCA) of two given nodes in the tree
+icon: line-dot-left-horizontal
 sidebar:
   label: 'Lowest Common Ancestor of a Binary Tree'
   badge: 'Medium'