docs(guides): add example how to parse query params for Bun.serve

[BitOfUniverse]

What does this PR do?

Guide describes all basic routing logic, but it doesn't tell anything about query params.
I've added a small snippet to make this guide complete.

• Documentation or TypeScript types (it's okay to leave the rest blank in this case)

[Araxeus]

1. Why the convoluted Object.fromEntries(searchParams.entries())?
   The example should be something basic like searchParams.get('token')

2. The example now creates a URL twice for no reason (new URL(req.url))