docs: add test examples with undici and fetch

[ghost]

Resolves #4296.

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[mcollina]

Note that you should keep keepAliveTimeout and keepAliveTimeoutMax set low (like 10): https://undici.nodejs.org/#/docs/api/Client?id=parameter-clientoptions. Otherwise in Node.js v16 and v14 the test will keep running for long.

(verify this!)

[mcollina]

lgtm

[L2jLiga]

lgtm

[jsumners]

If we must include the same example twice, the second example should have a preface describing why it is included.

[ghost]

@jsumners I added a description of the second example:
This example is a copy of the previous one, this time using fetch to make the request.