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

apps/docs/content/(05-decision-space)/intervals/435-non-overlapping-intervals.mdx

@@ -7,6 +7,7 @@ topics:
   - { name: "Dynamic Programming", slug: "dynamic-programming" }
   - { name: "Greedy", slug: "greedy" }
   - { name: "Sorting", slug: "sorting" }
+issue: "https://github.com/prdlk/leetcode/issues/183"
 sidebar:
   label: 'Non-overlapping Intervals'
   badge: 'Medium'

apps/docs/content/(05-decision-space)/intervals/56-merge-intervals.mdx

@@ -6,6 +6,7 @@ topics:
   - { name: "Array", slug: "array" }
   - { name: "Sorting", slug: "sorting" }
   - { name: "Quicksort", slug: "quicksort" }
+issue: "https://github.com/prdlk/leetcode/issues/182"
 sidebar:
   label: 'Merge Intervals'
   badge: 'Medium'

apps/docs/content/(05-decision-space)/intervals/57-insert-interval.mdx

@@ -4,6 +4,7 @@ description: You are given an array of non-overlapping intervals intervals where
 icon: dot
 topics:
   - { name: "Array", slug: "array" }
+issue: "https://github.com/prdlk/leetcode/issues/181"
 sidebar:
   label: 'Insert Interval'
   badge: 'Medium'