Update NoirJS tutorial in docs

[Savio-Sou]

Prerequisites

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1. Update to Noir v0.27.0 tiny-noirjs-app#4
   +0

Update the https://noir-lang.org/docs/tutorials/noirjs_app doc page per the changes in #4769.

[Savio-Sou]

On second read of the https://noir-lang.org/docs/tutorials/noirjs_app page:

@signorecello should engineers update it without updating the example, or should we count on @noir-lang/developerrelations to update both in one go?

[michaeljklein]

Summary of expected docs for new verifier class (could be added to the end of the existing noirjs_app page):

Change the BarretenbergBackend import line:

import { BarretenbergVerifierBackend } from '@noir-lang/backend_barretenberg';

and the setup line:

// try {
const backend = new BarretenbergVerifierBackend(circuit);

and then skip proving:

const proof = await noir.generateProof(input);

[Savio-Sou]

expected docs

Also verifyProof now takes in proof and verificationKey, instead of regenerating they verification key every time.

Adapted from the E2E test in #4769:

// Import
import { BarretenbergVerifier as Verifier } from '@noir-lang/backend_barretenberg';

// Generate vkey
const verificationKey = await backend.getVerificationKey();

// Verify proof
const verifier = new Verifier();
const isValid = await verifier.verifyProof(proof, verificationKey);