3.13.0 release notes: mention the new (upcoming) website

Author: michaelklishin

release-notes/3.13.0.md

@@ -207,6 +207,18 @@ on [our community Discord](https://rabbitmq.com/discord/).
 
 Release notes are kept under [rabbitmq-server/release-notes](https://github.com/rabbitmq/rabbitmq-server/tree/main/release-notes).
 
+### The Website
+
+This release, 3.13.0, introduces a major update to the RabbitMQ website.
+The new site includes:
+
+ * Versioning: over time, it will be able to provide documentation on a few most recent release seris, not just the latest one
+ * A reworked table of contents and navigation
+ * Search
+
+The update will go live around the 3.13.0 GA release date.
+
+
 ### Core Server
 
 #### Enhancements