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

apps/docs/content/(01-linear-structures)/stack/150-evaluate-reverse-polish-notation.mdx

@@ -7,6 +7,7 @@ topics:
   - { name: "Math", slug: "math" }
   - { name: "Stack", slug: "stack" }
 video: "iu0082c4HDE"
+issue: "https://github.com/prdlk/leetcode/issues/69"
 sidebar:
   label: 'Evaluate Reverse Polish Notation'
   badge: 'Medium'

apps/docs/content/(01-linear-structures)/stack/155-min-stack.mdx

@@ -6,6 +6,7 @@ topics:
   - { name: "Stack", slug: "stack" }
   - { name: "Design", slug: "design" }
 video: "qkLl7nAwDPo"
+issue: "https://github.com/prdlk/leetcode/issues/68"
 sidebar:
   label: 'Min Stack'
   badge: 'Medium'

apps/docs/content/(01-linear-structures)/stack/20-valid-parentheses.mdx

@@ -7,6 +7,7 @@ topics:
   - { name: "Stack", slug: "stack" }
   - { name: "Bracket Sequences", slug: "bracket-sequences" }
 video: "WTzjTskDFMg"
+issue: "https://github.com/prdlk/leetcode/issues/67"
 sidebar:
   label: 'Valid Parentheses'
   badge: 'Easy'