_content/doc: add reference on serving a module directly from a proxy
_content/doc: add reference on serving a module directly from a proxy

This was hinted at in "Finding a repository for a module path", but
this change makes the functionality more explicit and provides and
example.

Change-Id: I9a60c1d19c53108e8ae20112fcb3e88d97413677
Reviewed-on: https://go-review.googlesource.com/c/website/+/322670
Trust: Jay Conrod <f3fadaedc813344da5ec13b41e26596889cc5baa@google.com>
Reviewed-by: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>

golang/website,golang/website,golang/website,golang/website,golang/website,golang/website,golang/website