Link group/index.md to group-level project templates page

doc/user/group/index.md

@@ -259,6 +259,11 @@ types with every project in a group.
 
 Learn more about [Group-level file templates](https://docs.gitlab.com/ee/user/group/index.html#group-level-file-templates-premium).
 
+#### Group-level project templates **[PREMIUM]**
+
+Define project templates at a group-level by setting a group as a template source.
+[Learn more about group-level project templates.](custom_project_templates.md)
+
 ### Advanced settings
 
 - **Projects**: view all projects within that group, add members to each project,