update proxy docs to use NextProxy type (#90647)
### What?
Updated all proxy code examples in the API reference to use the
`NextProxy` typed arrow function pattern, consistent with the convention
used in `src/proxy.ts`.
### Why?
The proxy function is typed as `NextProxy` so parameters are inferred
from the type, removing the need for explicit `NextRequest` imports.
### How?
Updated all proxy code examples in the API reference to use the
`NextProxy` typed arrow function pattern, consistent with the convention
used in `src/proxy.ts`.
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Co-authored-by: Joseph <sephxd1234@gmail.com>
Co-authored-by: Joseph <joseph.chamochumbi@vercel.com>
