https://golang.org/design/DOC redirects to
https://github.com/golang/proposal/blob/master/design/DOC.md.

We should redirect it instead to
https://go.googlesource.com/proposal/+/master/design/DOC.md.

The Gerrit Markdown renderer appears to be just as capable as GitHub's,
the Gerrit repo is the actual source of truth,
changes made to the design repo are visible immediately in the Gerrit renderer instead of having to wait for GitHub mirroring,
and most important of all the Gerrit rendering is a well-formatted, printable document instead of just a web page.

Compare:

https://github.com/golang/proposal/blob/master/design/25719-go15vendor.md
https://go.googlesource.com/proposal/+/master/design/25719-go15vendor.md

/cc @andybons