docs: fix banner, adjust message (#878)

* docs: fix banner URL

* docs: banner message

Author: jbroma

website/theme/index.tsx

@@ -26,8 +26,8 @@ const Layout = () => (
   <Theme.Layout
     beforeNav={
       <Announcement
-        href="5.x/docs/getting-started/quick-start"
-        message="Re.Pack 5 with support for Rspack is coming!"
+        href="/5.x/docs/getting-started/quick-start"
+        message="Preview Re.Pack 5 RC documentation"
         localStorageKey="repack-announcement"
       />
     }