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

apps/docs/content/(02-nodal-grid)/linked-list/141-linked-list-cycle.mdx

@@ -1,6 +1,7 @@
 ---
 title: '141. Linked List Cycle'
 description: Given head, the head of a linked list, determine if the linked list has a cycle in it
+icon: line-dot-left-horizontal
 sidebar:
   label: 'Linked List Cycle'
   badge: 'Easy'

apps/docs/content/(02-nodal-grid)/linked-list/206-reverse-linked-list.mdx

@@ -1,6 +1,7 @@
 ---
 title: '206. Reverse Linked List'
 description: Given the head of a singly linked list, reverse the list, and return the reversed list
+icon: line-dot-left-horizontal
 sidebar:
   label: 'Reverse Linked List'
   badge: 'Easy'

apps/docs/content/(02-nodal-grid)/linked-list/21-merge-two-sorted-lists.mdx

@@ -1,6 +1,7 @@
 ---
 title: '21. Merge Two Sorted Lists'
 description: You are given the heads of two sorted linked lists list1 and list2
+icon: line-dot-left-horizontal
 sidebar:
   label: 'Merge Two Sorted Lists'
   badge: 'Easy'