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

apps/docs/content/(03-hierarchical)/binary-search-tree/230-kth-smallest-element-in-a-bst.mdx

@@ -1,6 +1,7 @@
 ---
 title: '230. Kth Smallest Element in a BST'
 description: Given the root of a binary search tree, and an integer k, return the k^th smallest value (1-indexed) of all the values of the nodes in the tree
+icon: line-dot-left-horizontal
 sidebar:
   label: 'Kth Smallest Element in a BST'
   badge: 'Medium'

apps/docs/content/(03-hierarchical)/binary-search-tree/235-lowest-common-ancestor-of-a-binary-search-tree.mdx

@@ -1,6 +1,7 @@
 ---
 title: '235. Lowest Common Ancestor of a Binary Search Tree'
 description: Given a binary search tree (BST), find the lowest common ancestor (LCA) node of two given nodes in the BST
+icon: line-dot-left-horizontal
 sidebar:
   label: 'Lowest Common Ancestor of a Binary Search Tree'
   badge: 'Medium'

apps/docs/content/(03-hierarchical)/binary-search-tree/98-validate-binary-search-tree.mdx

@@ -1,6 +1,7 @@
 ---
 title: '98. Validate Binary Search Tree'
 description: Given the root of a binary tree, determine if it is a valid binary search tree (BST)
+icon: line-dot-left-horizontal
 sidebar:
   label: 'Validate Binary Search Tree'
   badge: 'Medium'