feat(docs): add custom docs 404 page

apps/web/src/app/docs/not-found.tsx

@@ -0,0 +1,47 @@
+import { ArrowLeft, BookOpenText, House } from "lucide-react";
+import Link from "next/link";
+
+import { Button } from "@/components/ui/button";
+import {
+  Empty,
+  EmptyContent,
+  EmptyDescription,
+  EmptyHeader,
+  EmptyMedia,
+  EmptyTitle,
+} from "@/components/ui/empty";
+
+export default function DocsNotFound() {
+  return (
+    <div className="flex min-h-[calc(100dvh-var(--fd-nav-height))] items-center justify-center px-6 py-16">
+      <Empty className="border-border/60 bg-card/40 mx-auto max-w-xl rounded-2xl border border-dashed">
+        <EmptyHeader>
+          <EmptyMedia variant="icon">
+            <BookOpenText className="size-4" />
+          </EmptyMedia>
+          <EmptyTitle className="text-base sm:text-lg">Documentation page not found</EmptyTitle>
+          <EmptyDescription>
+            The docs page you requested does not exist or may have moved. Start from the docs index
+            or head back to the homepage.
+          </EmptyDescription>
+        </EmptyHeader>
+
+        <EmptyContent className="sm:flex-row sm:justify-center">
+          <Button render={<Link href="/docs/get-started" />} nativeButton={false}>
+            <BookOpenText />
+            Open docs
+          </Button>
+          <Button render={<Link href="/" />} nativeButton={false} variant="ghost">
+            <House />
+            Go home
+          </Button>
+        </EmptyContent>
+
+        <div className="text-muted-foreground flex items-center gap-2 text-xs">
+          <ArrowLeft className="size-3.5" />
+          Try the sidebar or search to find another page.
+        </div>
+      </Empty>
+    </div>
+  );
+}