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

apps/docs/content/(02-nodal-grid)/hybrid-structures/146-lru-cache.mdx

@@ -1,6 +1,7 @@
 ---
 title: '146. LRU Cache'
 description: Design a data structure that follows the constraints of a Least Recently Used (LRU) cache
+icon: line-dot-left-horizontal
 sidebar:
   label: 'LRU Cache'
   badge: 'Medium'

apps/docs/content/(02-nodal-grid)/hybrid-structures/2-add-two-numbers.mdx

@@ -1,6 +1,7 @@
 ---
 title: '2. Add Two Numbers'
 description: You are given two non-empty linked lists representing two non-negative integers. The digits are stored in reverse order, and each of their nodes contains a single digit. Add the two numbers and return the sum as a linked list
+icon: line-dot-left-horizontal
 sidebar:
   label: 'Add Two Numbers'
   badge: 'Medium'