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

apps/docs/content/(03-hierarchical)/tree-dfs/104-maximum-depth-of-binary-tree.mdx

@@ -8,6 +8,7 @@ topics:
   - { name: "Breadth-First Search", slug: "breadth-first-search" }
   - { name: "Binary Tree", slug: "binary-tree" }
 video: "hTM3phVI6YQ"
+issue: "https://github.com/prdlk/leetcode/issues/97"
 sidebar:
   label: 'Maximum Depth of Binary Tree'
   badge: 'Easy'

apps/docs/content/(03-hierarchical)/tree-dfs/226-invert-binary-tree.mdx

@@ -8,6 +8,7 @@ topics:
   - { name: "Breadth-First Search", slug: "breadth-first-search" }
   - { name: "Binary Tree", slug: "binary-tree" }
 video: "OnSn2XEQ4MY"
+issue: "https://github.com/prdlk/leetcode/issues/96"
 sidebar:
   label: 'Invert Binary Tree'
   badge: 'Easy'

apps/docs/content/(03-hierarchical)/tree-dfs/236-lowest-common-ancestor-of-a-binary-tree.mdx

@@ -8,6 +8,7 @@ topics:
   - { name: "Binary Tree", slug: "binary-tree" }
   - { name: "Binary Lifting", slug: "binary-lifting" }
   - { name: "Lowest Common Ancestor", slug: "lowest-common-ancestor" }
+issue: "https://github.com/prdlk/leetcode/issues/98"
 sidebar:
   label: 'Lowest Common Ancestor of a Binary Tree'
   badge: 'Medium'