docs: add deprecation note

Author: PaulRBerg

README.md

@@ -1,4 +1,7 @@
-# Sablier Docs
+# Sablier Legacy Docs
+
+> [!NOTE]
+> This repo has been superseded by [sablier-labs/docs](https://github.com/sablier-labs/docs). You can see the latest version of the Sablier documentation at [docs.sablier.com](https://docs.sablier.finance).
 
 Contributing to the Sablier docs is a great way to get involved in the dev community and help other people along the
 way.