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

apps/docs/content/(01-linear-structures)/hash-table/217-contains-duplicate.mdx

@@ -1,6 +1,7 @@
 ---
 title: '217. Contains Duplicate'
 description: Given an integer array nums, return true if any value appears at least twice in the array, and return false if every element is distinct
+icon: line-dot-left-horizontal
 sidebar:
   label: 'Contains Duplicate'
   badge: 'Easy'