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

apps/docs/content/(01-linear-structures)/two-pointers/344-reverse-string.mdx

@@ -1,6 +1,7 @@
 ---
 title: '344. Reverse String'
 description: Write a function that reverses a string. The input string is given as an array of characters s
+icon: line-dot-left-horizontal
 sidebar:
   label: 'Reverse String'
   badge: 'Easy'