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

apps/docs/content/(01-linear-structures)/prefix-sum/238-product-of-array-except-self.mdx

@@ -6,6 +6,7 @@ topics:
   - { name: "Array", slug: "array" }
   - { name: "Prefix Sum", slug: "prefix-sum" }
 video: "bNvIQI2wAjk"
+issue: "https://github.com/prdlk/leetcode/issues/37"
 sidebar:
   label: 'Product of Array Except Self'
   badge: 'Medium'

apps/docs/content/(01-linear-structures)/prefix-sum/303-range-sum-query-immutable.mdx

@@ -7,6 +7,7 @@ topics:
   - { name: "Design", slug: "design" }
   - { name: "Prefix Sum", slug: "prefix-sum" }
 video: "2pndAmo_sMA"
+issue: "https://github.com/prdlk/leetcode/issues/36"
 sidebar:
   label: 'Range Sum Query - Immutable'
   badge: 'Easy'

apps/docs/content/(01-linear-structures)/prefix-sum/560-subarray-sum-equals-k.mdx

@@ -7,6 +7,7 @@ topics:
   - { name: "Hash Table", slug: "hash-table" }
   - { name: "Prefix Sum", slug: "prefix-sum" }
 video: "fFVZt-6sgyo"
+issue: "https://github.com/prdlk/leetcode/issues/38"
 sidebar:
   label: 'Subarray Sum Equals K'
   badge: 'Medium'