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

apps/docs/content/(03-hierarchical)/binary-search-tree/230-kth-smallest-element-in-a-bst.mdx

@@ -8,6 +8,7 @@ topics:
   - { name: "Binary Search Tree", slug: "binary-search-tree" }
   - { name: "Binary Tree", slug: "binary-tree" }
 video: "5LUXSvjmGCw"
+issue: "https://github.com/prdlk/leetcode/issues/112"
 sidebar:
   label: 'Kth Smallest Element in a BST'
   badge: 'Medium'

apps/docs/content/(03-hierarchical)/binary-search-tree/235-lowest-common-ancestor-of-a-binary-search-tree.mdx

@@ -10,6 +10,7 @@ topics:
   - { name: "Binary Lifting", slug: "binary-lifting" }
   - { name: "Lowest Common Ancestor", slug: "lowest-common-ancestor" }
 video: "gs2LMfuOR9k"
+issue: "https://github.com/prdlk/leetcode/issues/113"
 sidebar:
   label: 'Lowest Common Ancestor of a Binary Search Tree'
   badge: 'Medium'

apps/docs/content/(03-hierarchical)/binary-search-tree/98-validate-binary-search-tree.mdx

@@ -8,6 +8,7 @@ topics:
   - { name: "Binary Search Tree", slug: "binary-search-tree" }
   - { name: "Binary Tree", slug: "binary-tree" }
 video: "s6ATEkipzow"
+issue: "https://github.com/prdlk/leetcode/issues/111"
 sidebar:
   label: 'Validate Binary Search Tree'
   badge: 'Medium'