docs: v4 banner

Author: amannn

docs/src/theme.config.tsx

@@ -27,21 +27,12 @@ export default {
     pre: Pre
   },
   banner: {
-    key: 'banner-learn-next-intl',
     content: (
       <div className="text-center">
-        Announcing{' '}
-        <a
-          className="underline"
-          href="https://learn.next-intl.dev"
-          rel="noreferrer"
-          target="_blank"
-        >
-          learn.next-intl.dev
-        </a>
-        !
+        You‘re viewing the next-intl 4 beta docs
       </div>
-    )
+    ),
+    dismissible: false
   },
   footer: {
     component: Footer