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

apps/docs/content/(01-linear-structures)/sliding-window/121-best-time-to-buy-and-sell-stock.mdx

@@ -1,6 +1,7 @@
 ---
 title: '121. Best Time to Buy and Sell Stock'
 description: You are given an array prices where prices[i] is the price of a given stock on the i^th day
+icon: line-dot-left-horizontal
 sidebar:
   label: 'Best Time to Buy and Sell Stock'
   badge: 'Easy'

apps/docs/content/(01-linear-structures)/sliding-window/3-longest-substring-without-repeating-characters.mdx

@@ -1,6 +1,7 @@
 ---
 title: '3. Longest Substring Without Repeating Characters'
 description: Given a string s, find the length of the longest substring without duplicate characters
+icon: line-dot-left-horizontal
 sidebar:
   label: 'Longest Substring Without Repeating Characters'
   badge: 'Medium'

apps/docs/content/(01-linear-structures)/sliding-window/424-longest-repeating-character-replacement.mdx

@@ -1,6 +1,7 @@
 ---
 title: '424. Longest Repeating Character Replacement'
 description: You are given a string s and an integer k. You can choose any character of the string and change it to any other uppercase English character. You can perform this operation at most k times
+icon: line-dot-left-horizontal
 sidebar:
   label: 'Longest Repeating Character Replacement'
   badge: 'Medium'

apps/docs/content/(01-linear-structures)/sliding-window/567-permutation-in-string.mdx

@@ -1,6 +1,7 @@
 ---
 title: '567. Permutation in String'
 description: Given two strings s1 and s2, return true if s2 contains a permutation of s1, or false otherwise
+icon: line-dot-left-horizontal
 sidebar:
   label: 'Permutation in String'
   badge: 'Medium'