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

apps/docs/content/(02-nodal-grid)/linked-list/141-linked-list-cycle.mdx

@@ -8,6 +8,7 @@ topics:
   - { name: "Two Pointers", slug: "two-pointers" }
   - { name: "Floyd's Cycle Finding Algorithm", slug: "floyds-cycle-finding-algorithm" }
 video: "gBTe7lFR3vc"
+issue: "https://github.com/prdlk/leetcode/issues/79"
 sidebar:
   label: 'Linked List Cycle'
   badge: 'Easy'

apps/docs/content/(02-nodal-grid)/linked-list/206-reverse-linked-list.mdx

@@ -6,6 +6,7 @@ topics:
   - { name: "Linked List", slug: "linked-list" }
   - { name: "Recursion", slug: "recursion" }
 video: "G0_I-ZF0S38"
+issue: "https://github.com/prdlk/leetcode/issues/77"
 sidebar:
   label: 'Reverse Linked List'
   badge: 'Easy'

apps/docs/content/(02-nodal-grid)/linked-list/21-merge-two-sorted-lists.mdx

@@ -6,6 +6,7 @@ topics:
   - { name: "Linked List", slug: "linked-list" }
   - { name: "Recursion", slug: "recursion" }
 video: "XIdigk956u0"
+issue: "https://github.com/prdlk/leetcode/issues/78"
 sidebar:
   label: 'Merge Two Sorted Lists'
   badge: 'Easy'