`$effect.active` docs (#9598)

* docs: $effect.active

* link

---------

Co-authored-by: Rich Harris <rich.harris@vercel.com>

sites/svelte-5-preview/src/routes/docs/content/01-api/02-runes.md

@@ -168,6 +168,24 @@ In rare cases, you may need to run code _before_ the DOM updates. For this we ca
 
 Previously, you would have used `beforeUpdate`, which — like `afterUpdate` — is deprecated in Svelte 5.
 
+## `$effect.active`
+
+The `$effect.active` rune is an advanced feature that tells you whether or not the code is running inside an effect or inside your template ([demo](/#H4sIAAAAAAAAE3XP0QrCMAwF0F-JRXAD595rLfgdzodRUyl0bVgzQcb-3VYFQfExl5tDMgvrPCYhT7MI_YBCiiOR2Aq-UxnSDT1jnlOcRlMSlczoiHUXOjYxpOhx5-O12rgAJg4UAwaGhDyR3Gxhjdai4V1v2N2wqus9tC3Y3ifMQjbehaqq4aBhLtEv_Or893icCsdLve-Caj8nBkU67zMO5HtGCfM3sKiWNKhV0zwVaBqd3x3ixVmHFyFLuJyXB-moOe8pAQAA)):
+
+```svelte
+<script>
+	console.log('in component setup:', $effect.active()); // false
+
+	$effect(() => {
+		console.log('in effect:', $effect.active()); // true
+	});
+</script>
+
+<p>in template: {$effect.active()}</p> <!-- true -->
+```
+
+This allows you to (for example) add things like subscriptions without causing memory leaks, by putting them in child effects.
+
 ## `$props`
 
 To declare component props, use the `$props` rune: