[docs] Replace 'Experimental APIs - Toolpad' with 'Toolpad (Beta)' (#44388)

Author: prakhargupta1

docs/data/material/components/snackbars/snackbars.md

@@ -141,7 +141,7 @@ The Snackbar component is composed of a root `<div>` that houses interior elemen
 </div>
 ```
 
-## Experimental APIs - Toolpad
+## Toolpad (Beta)
 
 ### useNotifications