feat: add tutorial-silent-auth-backend and update verify backend steps

[Kejmila]

added tutorial: https://github.com/alnacle/tutorial-silent-auth-backend/tree/main

added changes:
06-backend-start-verification.md (link) — remove the entire Coding Exercise section (it's a coding exercise with no correct answer provided):

Coding Exercise

We propose adding a new status endpoint for the mobile app.
This endpoint will query the verificationStore map and return the entry associated with the given request_id.

app.get("/status/:request_id", (req, res) => {
/** Your code goes here **/
});

The new method is especially useful for Silent Auth because the app could poll every 12 seconds for a short period instead of waiting blindly.2.
08-backend-callback-next.md (link) — in the "Configure the callback URL in the Vonage Dashboard" section, replace step 4:
Current:
4. Set it to https://your-codespace-url/callbackReplace with:
4. Take the URL you copied in step 3 above and add /callback at the end. Your final URL should look like this: https://your-codespace-name-3000.app.github.dev/callback
Add a note below the step 5:
Important: The URL is unique to your Codespace — do not copy the example above. Always use your own URL from the Ports tab with /callback appended.3.
09-rate-limiter.md (link) — delete the entire file (same issue as above — coding exercise with no correct answer provided)