Add webapp skeletons

matrixgw_frontend/src/routes/NotFoundRoute.tsx

@@ -0,0 +1,23 @@
+import { Button } from "@mui/material";
+import { RouterLink } from "../widgets/RouterLink";
+
+export function NotFoundRoute(): React.ReactElement {
+  return (
+    <div
+      style={{
+        textAlign: "center",
+        flex: "1",
+        display: "flex",
+        flexDirection: "column",
+        justifyContent: "center",
+        alignItems: "center",
+      }}
+    >
+      <h1>404 Not found</h1>
+      <p>The page you requested was not found!</p>
+      <RouterLink to="/">
+        <Button>Go back home</Button>
+      </RouterLink>
+    </div>
+  );
+}