add announcement banner

Author: skarim

docs/src/content.config.ts

@@ -1,10 +1,18 @@
-import { defineCollection } from 'astro:content';
+import { defineCollection, z } from 'astro:content';
 import { docsLoader } from '@astrojs/starlight/loaders';
 import { docsSchema } from '@astrojs/starlight/schema';
 
 export const collections = {
 	docs: defineCollection({
 		loader: docsLoader(),
-		schema: docsSchema(),
+		schema: docsSchema({
+			extend: z.object({
+				banner: z.object({
+					content: z.string(),
+				}).default({
+					content: 'Stacked PRs is currently in private preview. <a href="https://gh.io/stacksbeta">Sign up for the waitlist →</a>',
+				}),
+			}),
+		}),
 	}),
 };