Add validationMixin documentation

Add validation mixin on the doc (confirmation-message attribute not remove from solid-form and solid-delete pages yet).

source/import_documentation/Components/Solid-Delete.rst

@@ -16,6 +16,7 @@ Mixins
 This component uses the following mixin:
 
    * :ref:`next-mixin <next-mixin>`
+   * :ref:`validation-mixin <validation-mixin>`
 
 Please check their documentation to know more about its capabilities.