Document React Router Middleware Usage

### Proposed changes

[Middleware](https://reactrouter.com/how-to/middleware#middleware) is now stable in React Router framework mode. It requires a change to how custom server implementations (like `app/worker.ts` in the starter template) pass context to the request handler.

It would be _great_ to hear the CF team's official advice on wiring this up.

### Subject Matter

React Router middleware

### Content Location

an addition to the [React Router framework guide](https://developers.cloudflare.com/workers/framework-guides/web-apps/react-router/)

### Additional information

_No response_