docs: restructure problem paths, change section display to groups, and improve documentation content

apps/docs/content/(01-linear-structures)/prefix-sum/meta.ts

@@ -1,3 +1,3 @@
 import { defineMeta } from "blume";
 
-export default defineMeta({ title: "Prefix Sum", order: 2, icon: "sigma", display: "page" });
+export default defineMeta({ title: "Prefix Sum", order: 2, icon: "sigma", display: "group" });