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

apps/docs/content/(03-hierarchical)/heap-priority-queue/215-kth-largest-element-in-an-array.mdx

@@ -9,6 +9,7 @@ topics:
   - { name: "Heap (Priority Queue)", slug: "heap-priority-queue" }
   - { name: "Quickselect", slug: "quickselect" }
 video: "XEmy13g1Qxc"
+issue: "https://github.com/prdlk/leetcode/issues/118"
 sidebar:
   label: 'Kth Largest Element in an Array'
   badge: 'Medium'

apps/docs/content/(03-hierarchical)/heap-priority-queue/23-merge-k-sorted-lists.mdx

@@ -9,6 +9,7 @@ topics:
   - { name: "Merge Sort", slug: "merge-sort" }
   - { name: "Tournament Sort", slug: "tournament-sort" }
 video: "q5a5OiGbT6Q"
+issue: "https://github.com/prdlk/leetcode/issues/120"
 sidebar:
   label: 'Merge k Sorted Lists'
   badge: 'Hard'

apps/docs/content/(03-hierarchical)/heap-priority-queue/973-k-closest-points-to-origin.mdx

@@ -12,6 +12,7 @@ topics:
   - { name: "Quickselect", slug: "quickselect" }
   - { name: "K-D Tree", slug: "k-d-tree" }
 video: "rI2EBUEMfTk"
+issue: "https://github.com/prdlk/leetcode/issues/119"
 sidebar:
   label: 'K Closest Points to Origin'
   badge: 'Medium'