docs(docs): restructure documentation paths and add new introduction file

docs/02-arrays-and-strings/04-prefix-sum.mdx

@@ -0,0 +1,10 @@
+---
+title: 'Prefix Sum'
+description: 'Precomputed cumulative state for O(1) range queries.'
+sidebar:
+  icon: sticky-note
+---
+
+:::warning[Under construction]
+This page is a scaffold. Notes, canonical problems, and template code are not written yet.
+:::