docs: Deprecation of Middleware (#84710)
This PR removes middleware docs and adds a "Migration to Proxy" section
to the Proxy docs, which explains the rationale for the renaming to
Proxy and provides a migration guide.
Did not remove `middleware-upgrade-docs` as it's an upgrade doc from the
legacy middleware.
Below are untouched as they need codebase changes:
- `instrumentation` docs - `onRequestError` has `context.routeType` as
`'middleware'`
- `adapterPath` docs - has `MiddlewareMatcher` type example
- `ProxyConfig` - has a user-facing `MiddlewareMatcher` type
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Co-authored-by: Joseph Chamochumbi <joseph.chamochumbi@vercel.com>
