docs(content): add issue URLs to problem pages and restructure guides into appendix

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

@@ -6,6 +6,7 @@ topics:
   - { name: "Array", slug: "array" }
   - { name: "Dynamic Programming", slug: "dynamic-programming" }
 video: "1pkOgXD63yU"
+issue: "https://github.com/prdlk/leetcode/issues/50"
 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

@@ -7,6 +7,7 @@ topics:
   - { name: "String", slug: "string" }
   - { name: "Sliding Window", slug: "sliding-window" }
 video: "wiGpQwVHdE0"
+issue: "https://github.com/prdlk/leetcode/issues/51"
 sidebar:
   label: 'Longest Substring Without Repeating Characters'
   badge: 'Medium'

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

@@ -7,6 +7,7 @@ topics:
   - { name: "String", slug: "string" }
   - { name: "Sliding Window", slug: "sliding-window" }
 video: "gqXU1UyA8pk"
+issue: "https://github.com/prdlk/leetcode/issues/52"
 sidebar:
   label: 'Longest Repeating Character Replacement'
   badge: 'Medium'

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

@@ -8,6 +8,7 @@ topics:
   - { name: "String", slug: "string" }
   - { name: "Sliding Window", slug: "sliding-window" }
 video: "UbyhOgBN834"
+issue: "https://github.com/prdlk/leetcode/issues/53"
 sidebar:
   label: 'Permutation in String'
   badge: 'Medium'