The SvelteKit SSR example of +layout.svelte with invalidation call

See my issue supabase/auth-helpers#742

The SvelteKit documentation may be improved mainly by adding  src/routes/+layout.svelte example with invalidation call.

apps/docs/content/guides/auth/server-side/creating-a-client.mdx

@@ -462,7 +462,7 @@ export const handle: Handle = async ({ event, resolve }) => {
       /**
        * Note: You have to add the `path` variable to the
        * set and remove method due to sveltekit's cookie API
-       * requiring this to be set, setting the path to an empty string
+       * requiring this to be set, setting the path to '/'
        * will replicate previous/standard behaviour (https://kit.svelte.dev/docs/types#public-types-cookies)
        */
       set: (key, value, options) => {
@@ -552,6 +552,29 @@ export const load: LayoutServerLoad = async ({ locals: { getSession } }) => {
 
 </TabPanel>
 
+<TabPanel id="layout-svelte" label="Root Layout Page">
+
+```svelte +layout.svelte
+// src/routes/+layout.svelte
+<script>
+    import { invalidate } from '$app/navigation';
+    import { onMount } from 'svelte';
+
+    export let data;
+
+    $: ({ supabase } = data);
+
+    onMount(async () => {
+        supabase.auth.onAuthStateChange((event, _session) => {
+            invalidate('supabase:auth');
+        });
+        return () => subscription.unsubscribe();
+    });
+</script>
+```
+
+</TabPanel>
+
 <TabPanel id="page-component" label="Page Component">
 
 ```svelte +page.svelte