additional private preview callouts

Author: skarim

docs/src/content/docs/getting-started/quick-start.md

@@ -9,6 +9,10 @@ description: Install the gh stack CLI and create your first Stacked PR in minute
 - Git 2.20 or later
 - A GitHub repository you can push to
 
+:::caution[Private Preview]
+Stacked PRs is currently in private preview. This feature will **not work** unless enabled for your repository. [Sign up for the waitlist →](https://gh.io/stacksbeta)
+:::
+
 ## Install the CLI Extension
 
 ```sh

docs/src/content/docs/index.mdx

@@ -15,7 +15,7 @@ hero:
       icon: right-arrow
 ---
 
-import { Card, CardGrid } from '@astrojs/starlight/components';
+import { Card, CardGrid, Aside } from '@astrojs/starlight/components';
 import { Image } from 'astro:assets';
 import stackDiagram from '../../assets/stack-diagram.svg';
 import stackNavigator from '../../assets/screenshots/stack-navigator.png';
@@ -61,6 +61,10 @@ When you're ready to merge, you can merge all or a part of the stack. Each PR ca
 
 ## Get Started
 
+<Aside type="caution" title="Private Preview">
+Stacked PRs is currently in private preview. This feature will **not work** unless enabled for your repository. [Sign up for the waitlist →](https://gh.io/stacksbeta)
+</Aside>
+
 ```sh
 # Install the CLI extension
 gh extension install github/gh-stack