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

apps/docs/content/(01-linear-structures)/hash-table/1-two-sum.mdx

@@ -6,6 +6,7 @@ topics:
   - { name: "Array", slug: "array" }
   - { name: "Hash Table", slug: "hash-table" }
 video: "KLlXCFG5TnA"
+issue: "https://github.com/prdlk/leetcode/issues/29"
 sidebar:
   label: 'Two Sum'
   badge: 'Easy'

apps/docs/content/(01-linear-structures)/hash-table/217-contains-duplicate.mdx

@@ -7,6 +7,7 @@ topics:
   - { name: "Hash Table", slug: "hash-table" }
   - { name: "Sorting", slug: "sorting" }
 video: "3OamzN90kPg"
+issue: "https://github.com/prdlk/leetcode/issues/27"
 sidebar:
   label: 'Contains Duplicate'
   badge: 'Easy'

apps/docs/content/(01-linear-structures)/hash-table/242-valid-anagram.mdx

@@ -7,6 +7,7 @@ topics:
   - { name: "String", slug: "string" }
   - { name: "Sorting", slug: "sorting" }
 video: "9UtInBqnCgA"
+issue: "https://github.com/prdlk/leetcode/issues/28"
 sidebar:
   label: 'Valid Anagram'
   badge: 'Easy'

apps/docs/content/(01-linear-structures)/hash-table/347-top-k-frequent-elements.mdx

@@ -12,6 +12,7 @@ topics:
   - { name: "Counting", slug: "counting" }
   - { name: "Quickselect", slug: "quickselect" }
 video: "YPTqKIgVk-k"
+issue: "https://github.com/prdlk/leetcode/issues/31"
 sidebar:
   label: 'Top K Frequent Elements'
   badge: 'Medium'

apps/docs/content/(01-linear-structures)/hash-table/49-group-anagrams.mdx

@@ -8,6 +8,7 @@ topics:
   - { name: "String", slug: "string" }
   - { name: "Sorting", slug: "sorting" }
 video: "vzdNOK2oB2E"
+issue: "https://github.com/prdlk/leetcode/issues/30"
 sidebar:
   label: 'Group Anagrams'
   badge: 'Medium'