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

apps/docs/content/(04-relational)/graph-bfs/127-word-ladder.mdx

@@ -7,6 +7,7 @@ topics:
   - { name: "String", slug: "string" }
   - { name: "Breadth-First Search", slug: "breadth-first-search" }
   - { name: "Bidirectional Search", slug: "bidirectional-search" }
+issue: "https://github.com/prdlk/leetcode/issues/138"
 sidebar:
   label: 'Word Ladder'
   badge: 'Hard'

apps/docs/content/(04-relational)/graph-bfs/994-rotting-oranges.mdx

@@ -6,6 +6,7 @@ topics:
   - { name: "Array", slug: "array" }
   - { name: "Breadth-First Search", slug: "breadth-first-search" }
   - { name: "Matrix", slug: "matrix" }
+issue: "https://github.com/prdlk/leetcode/issues/137"
 sidebar:
   label: 'Rotting Oranges'
   badge: 'Medium'