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

apps/docs/content/(05-decision-space)/backtracking/39-combination-sum.mdx

@@ -1,6 +1,7 @@
 ---
 title: '39. Combination Sum'
 description: Given an array of distinct integers candidates and a target integer target, return a list of all unique combinations of candidates where the chosen numbers sum to target. You may return the combinations in any order
+icon: line-dot-left-horizontal
 sidebar:
   label: 'Combination Sum'
   badge: 'Medium'

apps/docs/content/(05-decision-space)/backtracking/46-permutations.mdx

@@ -1,6 +1,7 @@
 ---
 title: '46. Permutations'
 description: Given an array nums of distinct integers, return all the possible permutations. You can return the answer in any order
+icon: line-dot-left-horizontal
 sidebar:
   label: 'Permutations'
   badge: 'Medium'

apps/docs/content/(05-decision-space)/backtracking/78-subsets.mdx

@@ -1,6 +1,7 @@
 ---
 title: '78. Subsets'
 description: Given an integer array nums of unique elements, return all possible subsets (the power set)
+icon: line-dot-left-horizontal
 sidebar:
   label: 'Subsets'
   badge: 'Medium'