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

apps/docs/content/(01-linear-structures)/two-pointers/15-3sum.mdx

@@ -1,6 +1,7 @@
 ---
 title: '15. 3Sum'
 description: Given an integer array nums, return all the triplets [nums[i], nums[j], nums[k]] such that i != j, i != k, and j != k, and nums[i] + nums[j] + nums[k] == 0
+icon: line-dot-left-horizontal
 sidebar:
   label: '3Sum'
   badge: 'Medium'