docs(nextjs): add config for standalone output (#292)

Co-authored-by: Julius Marminge <[email protected]>

docs/src/app/docs/nextjs/page.mdx

@@ -105,6 +105,24 @@ export const env = createEnv({
 
 </Callout>
 
+<Callout type="info">
+
+If you're using the `standalone` output in your `next.config.ts`, make sure to include the following:  
+
+```ts title="next.config.ts"
+import type { NextConfig } from "next"
+
+const nextConfig: NextConfig = {
+  output: "standalone",
+  // Add the packages in transpilePackages
+  transpilePackages: ["@t3-oss/env-nextjs", "@t3-oss/env-core"],
+}
+
+export default nextConfig
+```
+
+</Callout>
+
 ### Validate schema on build (recommended)
 
 We recommend you importing your newly created file in your `next.config.js`. This will make sure your environment variables are validated at build time which will save you a lot of time and headaches down the road. You can use [unjs/jiti](https://github.com/unjs/jiti) to import TypeScript files in your `next.config.js`: