docs: Replace Middleware docs to Proxy (#84709)
> [!NOTE]
> Best reviewed by each commit for better diff view.
This PR clones the Middleware docs for Proxy and removes the Middleware docs.
Did not clone the list of docs:
- `errors/middleware-upgrade.mdx` - It's a middleware upgrade guide from v12.2
- `errors/beta-middleware.mdx` - It's an error when using middleware before v12.2
- `errors/returning-response-body.mdx` - Legacy behavior from versions < v12.2
