[docs] Document middleware response changes

[ethomson]

Beginning in Next.js 13, middleware can now respond directly, without needing to rewrite or redirect, when the experimental .allowMiddlewareResponseBody configuration setting is set. Document this behavior.

Bug

• [ ] Related issues linked using fixes #number
• [ ] Integration tests added
• [ ] Errors have a helpful link attached, see contributing.md

Feature

• [ ] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
• [ ] Related issues linked using fixes #number
• [ ] Integration tests added
• [ ] Documentation added
• [ ] Telemetry added. In case of a feature if it's used or not.
• [ ] Errors have a helpful link attached, see contributing.md

Documentation / Examples

• [x] Make sure the linting passes by running pnpm build && pnpm lint
• [ ] The "examples guidelines" are followed from our contributing doc