Add docs (or potentially a recipe) for proxying requests from an endpoint

[bholmesdev]

📚 Subject area/topic

SSR

📋 Suggested page

https://docs.astro.build/en/guides/endpoints/#server-endpoints-api-routes

📋 General description or bullet points (if proposing new content)

We received a roadmap discussion for adding better proxy support from API endpoints. Based on the replies, this is possible today by returning the result of a fetch() call. This capability is not documented, nor is it very intuitive. Would be worth an explicit guide on Proxying Requests.

🖥️ Reproduction of code samples in StackBlitz

No response