Replace infinite redirect with placeholder page (#10300)

content/registry.md

@@ -1,3 +1,20 @@
 ---
-redirect_to: /registry/
+title: "Pulumi Registry"
+readme: |
+    This page is intentionally left blank, which may seem strange, but is actually necessary because
+    of how the CloudFront CDN is configured to serve https://www.pulumi.com/registry/.
+
+    If we didn't put this page here, requests for "/registry" (sans trailing slash) would prompt S3 to
+    go looking in the currently deployed bucket for an object named `registry` (which it'd fail to find,
+    because such an object wouldn't exist), then go looking for an object named `registry/index.html`
+    (because `index.html` is defined as the bucket's `indexDocument`). On failing to find both, S3 would
+    return the bucket's `errorDocument` (i.e., our 404 page) and an error code to CloudFront instead --
+    which CloudFront would finally return to the user.
+
+    Instead, by including this document, we give S3 an object to deliver to Cloudfront (our intentionally
+    blank `registry/index.html`), but allow CloudFront to forward the request on to the Registry, which is
+    configured to handle all requests for `/registry/*` (including `/registry/index.html`).
+
+    More info here:
+    https://docs.aws.amazon.com/AmazonCloudFront/latest/DeveloperGuide/DefaultRootObject.html#DefaultRootObjectHow
 ---