Note Blankslate in empty state docs, show 3 WPs

Mentions that `with_empty_state` renders a `Primer::Beta::Blankslate`
and accepts `title:`, `description:`, and `icon:` keywords. Bumps the
work-package preview from 2 to 3 items for visual consistency with the
other preview scenarios.

lookbook/docs/components/border-box-list.md.erb

@@ -112,7 +112,9 @@ for card-specific layout, parameters, and slots.
 
 ### Empty state
 
-Use `with_empty_state` when the list can be rendered without items.
+Use `with_empty_state` when the list can be rendered without items. It renders a
+`Primer::Beta::Blankslate` under the hood, accepting `title:`, `description:`,
+and `icon:` keywords.
 
 <%= embed OpenProject::Common::BorderBoxListComponentPreview, :empty_state, panels: %i[source] %>
 

lookbook/previews/open_project/common/border_box_list_component_preview.rb

@@ -129,7 +129,7 @@ module OpenProject
         interactive: false,
         collapsible: false
       )
-        work_packages = WorkPackage.includes(:project).limit(2).to_a
+        work_packages = WorkPackage.includes(:project).limit(3).to_a
         return preview_message("No work packages in the database.") if work_packages.empty?
 
         render OpenProject::Common::BorderBoxListComponent.new(