add changelog

Author: nadar

CHANGELOG.md

@@ -6,6 +6,7 @@ In order to read more about upgrading and BC breaks have a look at the [UPGRADE
 ## 5.1.1
 
 + [#410](https://github.com/luyadev/luya-module-cms/pull/410) Disabled sorting functionality for the "group" extra field in the block CRUD interface due to an exception being thrown. This issue occurred because the field is declared as an `extraAttribute`.
++ [#409](https://github.com/luyadev/luya-module-cms/issues/409) Implemented a new validation check to prevent slug duplication within the same language and navigation hierarchy when creating a new page. This enhancement ensures unique page identification and avoids conflicts in site structure.
 
 ## 5.1.0 (7. February 2024)