gddo-server: add initial redirect logic

Logic is added to redirect specific paths to pkg.go.dev.

Change-Id: I49b3d446e99c5684dd9121322b822f84b1cda1be
Reviewed-on: https://go-review.googlesource.com/c/gddo/+/281732
Trust: Julie Qiu <julie@golang.org>
Run-TryBot: Julie Qiu <julie@golang.org>
Reviewed-by: Jonathan Amsterdam <jba@google.com>