proxy.golang.org: Make 302 Redirect to Google Cloud Storage Explicit

[njgibbon]

Hello,

Sometimes the proxy will redirect to Google Cloud Storage for archive downloads.

As mentioned here: #33928

This issue is no biggie. But having looked around the documentation I can not find any reference to the proxy working like this. Given that this is a technical tooling system and not a general consumer service I think the dependency on the external service should be made explicit somewhere.

Certain environments (including the Go Teams own development environments from linked issue) require domain allowlists and this information would help in those cases.

Looking around the internet at various sources of how the go proxy and module downloading works there is not general knowledge of the redirect part of the system.

[mknyszek]

CC @golang/tools-team

[hyangah]

Multiple users were looking for this info. We need to document it (and also info shown in #49144 (comment) to help users setting up egress rules)