Document `hide_version` flag (#33670)

Unauthenticated users can be barred from being shown the current Grafana server version since https://github.com/grafana/grafana/pull/24919

docs/sources/auth/grafana.md

@@ -65,6 +65,9 @@ org_name = Main Org.
 
 # Role for unauthenticated users, other valid values are `Editor` and `Admin`
 org_role = Viewer
+
+# Hide the Grafana version text from the footer and help tooltip for unauthenticated users (default: false)
+hide_version = true
 ```
 
 If you change your organization name in the Grafana UI this setting needs to be updated to match the new name.