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

apps/docs/content/appendix/(3-techniques)/combinatorics.mdx

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+---
+title: "Combinatorics"
+list: "https://leetcode.com/problem-list/combinatorics/"
+icon: dices
+sidebar:
+  badge: '1'
+---
+
+:::note
+This guide has not been written yet. The LeetCode list above has every problem tagged Combinatorics.
+:::

apps/docs/content/appendix/(3-techniques)/counting.mdx

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+---
+title: "Counting"
+list: "https://leetcode.com/problem-list/counting/"
+icon: tally-5
+sidebar:
+  badge: '3'
+---
+
+:::note
+This guide has not been written yet. The LeetCode list above has every problem tagged Counting.
+:::

apps/docs/content/appendix/(3-techniques)/design.mdx

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+---
+title: "Design"
+list: "https://leetcode.com/problem-list/design/"
+icon: pencil-ruler
+sidebar:
+  badge: '5'
+---
+
+:::note
+This guide has not been written yet. The LeetCode list above has every problem tagged Design.
+:::

apps/docs/content/appendix/(3-techniques)/geometry.mdx

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+---
+title: "Geometry"
+list: "https://leetcode.com/problem-list/geometry/"
+icon: shapes
+sidebar:
+  badge: '1'
+---
+
+:::note
+This guide has not been written yet. The LeetCode list above has every problem tagged Geometry.
+:::

apps/docs/content/appendix/(3-techniques)/math.mdx

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+---
+title: "Math"
+list: "https://leetcode.com/problem-list/math/"
+icon: calculator
+sidebar:
+  badge: '6'
+---
+
+:::note
+This guide has not been written yet. The LeetCode list above has every problem tagged Math.
+:::

apps/docs/content/appendix/(3-techniques)/meta.ts

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+import { defineMeta } from "blume";
+
+export default defineMeta({ title: "Techniques", order: 3 });