docs(docs): restructure and expand documentation with new phases and content

docs/03-guides/02-two-pointers.mdx

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+---
+title: 'Two Pointers'
+description: 'Converging/parallel index walk that prunes the O(n^2) pair space using order.'
+---
+
+:::warning[Under construction]
+This page is a scaffold. Notes, canonical problems, and template code are not written yet.
+:::
+
+## Concepts
+
+
+
+
+## Problems
+
+<CardGroup cols={2}>
+  <Card title="344. Reverse String" href="/problems/reverse-string">
+  </Card>
+  <Card title="977. Squares of a Sorted Array" href="/problems/squares-of-a-sorted-array">
+  </Card>
+</CardGroup>