Add app router example (#52066)

- Add app router example to 03-environment-variables.mdx

Co-authored-by: Delba de Oliveira <32464864+delbaoliveira@users.noreply.github.com>
Co-authored-by: Steven <229881+styfle@users.noreply.github.com>

docs/02-app/01-building-your-application/06-configuring/03-environment-variables.mdx

@@ -48,9 +48,20 @@ export async function getStaticProps() {
 
 <AppOnly>
 
-This loads `process.env.DB_HOST`, `process.env.DB_USER`, and `process.env.DB_PASS` into the Node.js environment automatically allowing you to use them in Route Handlers.
+This loads `process.env.DB_HOST`, `process.env.DB_USER`, and `process.env.DB_PASS` into the Node.js environment automatically allowing you to use them in [Route Handlers](/docs/app/building-your-application/routing/router-handlers).
 
-{/* TODO: App Router Example */}
+For example:
+
+```js filename="app/api/route.js"
+export async function GET() {
+  const db = await myDB.connect({
+    host: process.env.DB_HOST,
+    username: process.env.DB_USER,
+    password: process.env.DB_PASS,
+  })
+  // ...
+}
+```
 
 </AppOnly>