docs: add guides (#258)

apps/public/content/docs/(tracking)/sdks/express.mdx

@@ -4,11 +4,16 @@ description: The Express middleware is a basic wrapper around Javascript SDK. It
 ---
 
 import Link from 'next/link';
+import { Callout } from 'fumadocs-ui/components/callout';
 import { DeviceIdWarning } from '@/components/device-id-warning';
 import { PersonalDataWarning } from '@/components/personal-data-warning';
 
 import CommonSdkConfig from '@/components/common-sdk-config.mdx';
 
+<Callout>
+Looking for a step-by-step tutorial? Check out the [Express analytics guide](/guides/express-analytics).
+</Callout>
+
 ## Installation
 
 ```bash