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

apps/docs/content/(02-nodal-grid)/hybrid-structures/146-lru-cache.mdx

@@ -8,6 +8,7 @@ topics:
   - { name: "Design", slug: "design" }
   - { name: "Doubly-Linked List", slug: "doubly-linked-list" }
 video: "7ABFKPK2hD4"
+issue: "https://github.com/prdlk/leetcode/issues/87"
 sidebar:
   label: 'LRU Cache'
   badge: 'Medium'

apps/docs/content/(02-nodal-grid)/hybrid-structures/2-add-two-numbers.mdx

@@ -7,6 +7,7 @@ topics:
   - { name: "Math", slug: "math" }
   - { name: "Recursion", slug: "recursion" }
 video: "wgFPrzTjm7s"
+issue: "https://github.com/prdlk/leetcode/issues/88"
 sidebar:
   label: 'Add Two Numbers'
   badge: 'Medium'