[Backport 13.4] Add site set warning message

Documentation/CreatePages/Index.rst

@@ -54,6 +54,14 @@ The site configuration is stored in a file called
 :file:`config/sites/main/config.yaml`. You can edit this file in the
 backend module :guilabel:`Site Management > Sites`.
 
+When the :guilabel:`Sites` module is loaded, a warning will be displayed, which will disappear when our package site is created during this tutorial.
+
+..  figure:: SiteSetDependencieWarning.png
+    :class: with-shadow
+    :alt: Screenshot of the backend module "Site" with a warning related to missing site set
+
+Once this package has been created, the screen will look like this:
+
 ..  figure:: SiteConfiguration.png
     :alt: Screenshot of the backend module "Site Management"