_content: add AUTHORS page, and CONTRIBUTORS redirect

Now that there's no AUTHORS file, go.dev/AUTHORS will become a dead link
as of Go 1.19, and tip.golang.org/AUTHORS is already one.
Write a page to keep those links not 404-ing.

Same is true for CONTRIBUTORS; redirect to AUTHORS.

For golang/go#53961.

Change-Id: I17cc932f3f870bf4b9970151ead4b5e72a94845b
Reviewed-on: https://go-review.googlesource.com/c/website/+/418932
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

_content/AUTHORS.md

@@ -0,0 +1,14 @@
+---
+title: "Authors of Go"
+layout: article
+date: 2022-07-25T10:15:00-04:00
+---
+
+The Go project formerly maintained AUTHORS and CONTRIBUTORS files
+listing the authors of commits in the Go project repositories.
+The source control history is now the authoritative source for
+this information.
+
+Go's Gerrit instance can search for changes by specific authors.
+For example \[[is:merged author:"Russ Cox"](https://go-review.googlesource.com/q/is:merged+author:%2522Russ+Cox%2522)\].
+

_content/CONTRIBUTORS.md

@@ -0,0 +1,3 @@
+---
+redirect: /AUTHORS
+---

cmd/golangorg/testdata/web.txt

@@ -428,3 +428,9 @@ redirect == /doc/#references
 
 GET https://go.dev/ref/
 redirect == /doc/#references
+
+GET https://go.dev/AUTHORS
+body contains is:merged author:
+
+GET https://go.dev/CONTRIBUTORS
+redirect == /AUTHORS