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

apps/docs/content/(01-linear-structures)/stack/20-valid-parentheses.mdx

@@ -1,6 +1,7 @@
 ---
 title: '20. Valid Parentheses'
 description: Given a string s containing just the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid
+icon: line-dot-left-horizontal
 sidebar:
   label: 'Valid Parentheses'
   badge: 'Easy'