add the middleware.ts API reference file convention (#61037)
This PR adds a new docs page under the File Conventions section for
the`middleware.ts` file. It includes:
- Explanation of `middleware.ts` usage in Next.js.
- Code examples in TypeScript and JavaScript.
- Information on `matcher` function, `NextResponse`, and middleware
runtime environment.
- Version history of middleware feature updates.
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Co-authored-by: Delba de Oliveira <32464864+delbaoliveira@users.noreply.github.com>
Co-authored-by: Michael Novotny <manovotny@gmail.com>
