doc: use new Gerrit URL and mention our instance in CONTRIBUTING.md

Fixes #14662

Change-Id: I8f7d5e8d2dd8dc763e940e061b424427359246e7
Reviewed-on: https://go-review.googlesource.com/20462Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

CONTRIBUTING.md

@@ -26,7 +26,8 @@ Please read the [Contribution Guidelines](https://golang.org/doc/contribute.html
 before sending patches.
 
 **We do not accept GitHub pull requests**
-(we use [Gerrit](https://code.google.com/p/gerrit/) instead for code review).
+(we use [an instance](https://go-review.googlesource.com/) of the
+[Gerrit](https://www.gerritcodereview.com/) code review system instead).
 
 Unless otherwise noted, the Go source files are distributed under
 the BSD-style license found in the LICENSE file.

doc/contribute.html

@@ -78,7 +78,7 @@ no matter who makes the change.
 A custom git command called <code>git-codereview</code>,
 discussed below, helps manage the code review process through a Google-hosted
 <a href="https://go-review.googlesource.com/">instance</a> of the code review
-system called <a href="https://code.google.com/p/gerrit/">Gerrit</a>.
+system called <a https://www.gerritcodereview.com/">Gerrit</a>.
 </p>
 
 <h3 id="auth">Set up authentication for code review</h3>