Implement 404 page

Author: rikurauhala

src/app/not-found.tsx

@@ -0,0 +1,21 @@
+import { Container, Link, Stack, Typography } from "@mui/material";
+import NextLink from "next/link";
+
+import { Toskamopo } from "@/components/Toskamopo";
+
+const NotFound = () => {
+  return (
+    <Container maxWidth="md">
+      <Stack gap={2} sx={{ alignItems: "center", py: 4 }}>
+        <Toskamopo />
+        <Typography variant="h2">404</Typography>
+        <Typography variant="body1">Sivua ei löytynyt</Typography>
+        <Link component={NextLink} href="/">
+          Takaisin etusivulle
+        </Link>
+      </Stack>
+    </Container>
+  );
+};
+
+export default NotFound;

src/components/AboutUs/index.tsx

@@ -3,7 +3,7 @@
 import { MarkdownContainer } from "@/components/MarkdownContainer";
 import { Stack } from "@mui/material";
 
-import { Toskamopo } from "./Toskamopo";
+import { Toskamopo } from "@/components/Toskamopo";
 
 export const AboutUs = async ({ introText }: { introText: string }) => {
   return (