Make data from related_to_step_nav property to be accessisible

This is required because when a content item has step by step disabled
it does not have the step_navs property but has the
step by steps within related_to_step_navs.

This is needed for the personalisation work that will be done to show step
by steps on pages where we know the user is actively interacting with a
specific step by step.

lib/govuk_publishing_components/presenters/page_with_step_by_step_navigation.rb

@@ -15,6 +15,12 @@ def step_navs
         end
       end
 
+      def related_to_step_navs
+        @related_to_step_navs ||= parsed_related_to_step_navs.map do |step_nav|
+          StepByStepModel.new(step_nav)
+        end
+      end
+
       def show_sidebar?
         show_header? && current_step_nav.steps.present?
       end
@@ -92,6 +98,10 @@ def parsed_step_navs
         content_item.dig("links", "part_of_step_navs").to_a
       end
 
+      def parsed_related_to_step_navs
+        content_item.dig("links", "related_to_step_navs").to_a
+      end
+
       def configure_for_sidebar(step_nav_content)
         step_nav_content[:steps].each_with_index do |step, step_index|
           step[:contents].each do |content|