Update CNAME to redirect docs too (#15046)

diff --git a/docs/website/docs/CNAME b/docs/website/docs/CNAME
index 4cc6a37..dc1f918 100644
--- a/docs/website/docs/CNAME
+++ b/docs/website/docs/CNAME
@@ -1 +1,2 @@
 www.iree.dev
+docs.iree.dev