docs(push): add test push subsection to step 3

Covers sending test pushes via Ably CLI by client ID and device ID
once the device is activated, before moving on to channel subscriptions.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: maratal

src/pages/docs/push/getting-started/nextjs.mdx

@@ -297,6 +297,29 @@ npm run dev
 
 Navigate to `http://localhost:3000/push` in your browser. Click **Activate Push**, grant notification permission, and wait for the device ID to appear.
 
+### Test push notifications <a id="step-3-test"/>
+
+Once the device is activated, send a test push notification directly to your client ID using the Ably CLI:
+
+<Code>
+```shell
+ably push publish --client-id push-tutorial-client \
+  --title "Test push" \
+  --body "Hello from CLI!" \
+  --data '{"foo":"bar"}'
+```
+</Code>
+
+A native browser notification should appear and the log should display the received push event. You can also send to the specific device ID shown in the log:
+
+<Code>
+```shell
+ably push publish --device-id <your-device-id> \
+  --title "Test push" \
+  --body "Hello from CLI!"
+```
+</Code>
+
 ![Screenshot of the Next.js push tutorial application showing push notifications received](../../../../images/content/screenshots/getting-started/nextjs-push-getting-started-guide.png)
 
 ## Step 4: Subscribe to channel push <a id="step-4"/>