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

apps/docs/content/(05-decision-space)/backtracking/39-combination-sum.mdx

@@ -5,6 +5,7 @@ icon: dot
 topics:
   - { name: "Array", slug: "array" }
   - { name: "Backtracking", slug: "backtracking" }
+issue: "https://github.com/prdlk/leetcode/issues/150"
 sidebar:
   label: 'Combination Sum'
   badge: 'Medium'

apps/docs/content/(05-decision-space)/backtracking/46-permutations.mdx

@@ -5,6 +5,7 @@ icon: dot
 topics:
   - { name: "Array", slug: "array" }
   - { name: "Backtracking", slug: "backtracking" }
+issue: "https://github.com/prdlk/leetcode/issues/151"
 sidebar:
   label: 'Permutations'
   badge: 'Medium'

apps/docs/content/(05-decision-space)/backtracking/78-subsets.mdx

@@ -6,6 +6,7 @@ topics:
   - { name: "Array", slug: "array" }
   - { name: "Backtracking", slug: "backtracking" }
   - { name: "Bit Manipulation", slug: "bit-manipulation" }
+issue: "https://github.com/prdlk/leetcode/issues/149"
 sidebar:
   label: 'Subsets'
   badge: 'Medium'