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

apps/docs/content/appendix/(1-data-structures)/binary-tree.mdx

@@ -0,0 +1,11 @@
+---
+title: "Binary Tree"
+list: "https://leetcode.com/problem-list/binary-tree/"
+icon: git-branch
+sidebar:
+  badge: '9'
+---
+
+:::note
+This guide has not been written yet. The LeetCode list above has every problem tagged Binary Tree.
+:::