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

apps/docs/content/(01-linear-structures)/hash-table/1-two-sum.mdx

@@ -1,6 +1,7 @@
 ---
 title: '1. Two Sum'
 description: You are given an array of integers nums and an integer target, return indices of the two numbers such that they add up to target
+icon: line-dot-left-horizontal
 sidebar:
   label: 'Two Sum'
   badge: 'Easy'