Add notification banner to primary nav component

This aims to alert users that the Primary navigation component will be removed in future, and users should use the Service navigation component from the GOV.UK Design System instead.

docs/primary-navigation.md

@@ -6,6 +6,11 @@ description: Link to the primary sections of your service.
 ---
 
 {% from "example/macro.njk" import appExample %}
+{% from "govuk/components/notification-banner/macro.njk" import govukNotificationBanner %}
+
+{{ govukNotificationBanner({
+  html: "### This component will be removed in future\n\nUse the [service navigation component](https://design-system.service.gov.uk/components/service-navigation/) in the GOV.UK Design System instead." | markdown
+}) }}
 
 {{ appExample({
   example: "primary-navigation"