chore(docs): add express credentials example (#11097)

Co-authored-by: Kawahara Shotaro <[email protected]>
Co-authored-by: Nico Domino <[email protected]>

docs/pages/getting-started/authentication/credentials.mdx

@@ -102,6 +102,50 @@ export { handle } from "./auth"
 ```
 
 </Code.Svelte>
+
+<Code.Express>
+
+```ts filename="./src/routes/auth.route.ts" {2, 11}
+import { ExpressAuth } from '@auth/express'
+import Credentials from '@auth/express/providers/credentials'
+import express from "express"
+// Your own logic for dealing with plaintext password strings; be careful!
+import { saltAndHashPassword } from "@/utils/password"
+
+const app = express()
+app.use("/auth/*", ExpressAuth({
+  providers: [
+    Credentials({
+      // You can specify which fields should be submitted, by adding keys to the `credentials` object.
+      // e.g. domain, username, password, 2FA token, etc.
+      credentials: {
+        email: {},
+        password: {},
+      },
+      authorize: async (credentials) => {
+        let user = null
+
+        // logic to salt and hash password
+        const pwHash = saltAndHashPassword(credentials.password)
+
+        // logic to verify if user exists
+        user = await getUserFromDb(credentials.email, pwHash)
+
+        if (!user) {
+          // No user found, so this is their first attempt to login
+          // meaning this is also the place you could do registration
+          throw new Error("User not found.")
+        }
+
+        // return user object with the their profile data
+        return user
+      },
+    }),
+  ],
+}))
+```
+
+</Code.Express>
 </Code>
 
 <Callout type="info">