feat(error): add custom 404-page

tavla/app/not-found.tsx

@@ -0,0 +1,24 @@
+import { Button } from '@entur/button'
+import { Heading3 } from '@entur/typography'
+import Link from 'next/link'
+import Image from 'next/image'
+import BeaverIllustration from 'assets/illustrations/Beaver.png'
+
+function Custom404() {
+    return (
+        <div className="mx-auto h-[70vh] lg:w-1/4 flex flex-col justify-center items-center gap-5">
+            <Heading3>Denne siden finnes ikke!</Heading3>
+            <Image
+                src={BeaverIllustration}
+                className="w-1/2"
+                alt="Illustrasjon av en bever"
+            />
+
+            <Button as={Link} href="/" variant="primary">
+                Tilbake til forsiden
+            </Button>
+        </div>
+    )
+}
+
+export default Custom404

tavla/pages/404.tsx

@@ -1,14 +1,24 @@
-import { Header } from 'components/Header'
+import { Button } from '@entur/button'
+import { Heading3 } from '@entur/typography'
+import Link from 'next/link'
+import Image from 'next/image'
+import BeaverIllustration from 'assets/illustrations/Beaver.png'
 
-function NotFoundPage() {
+function Custom404() {
     return (
-        <div className="root">
-            <div className="rootContainer">
-                <Header />
-                <p className="pl-8">Tavlen kunne ikke hentes!</p>
-            </div>
+        <div className="mx-auto h-[70vh] lg:w-1/4 flex flex-col justify-center items-center gap-5">
+            <Heading3>Denne siden finnes ikke!</Heading3>
+            <Image
+                src={BeaverIllustration}
+                className="w-1/2"
+                alt="Illustrasjon av en bever"
+            />
+
+            <Button as={Link} href="/" variant="primary">
+                Tilbake til forsiden
+            </Button>
         </div>
     )
 }
 
-export default NotFoundPage
+export default Custom404