Update README to reflect archive status and redirect users

The README now indicates the repository has been archived and will no longer receive updates. A link to the new documentation repository is provided for users to access the latest resources.

Author: osmontero

README.md

@@ -1 +1,5 @@
-# UTMStack Documentation
+# UTMStack Documentation (Archived)
+
+This repository hosted the source code for docs.utmstack.com, but has been **archived**.  It will no longer receive updates.
+
+For the latest documentation, please visit the new repository: [https://github.com/utmstack/Docs](https://github.com/utmstack/Docs)