Add redirect stub for old page

Create a redirect stub for the alert notifications page, since we
DO still need the old redirect style.

doc/operations/incident_management/alert_notifications.md

+---
+redirect_to: 'paging.md'
+---
+
+This document was moved to [another location](paging.md).
+
+<!-- This redirect file can be deleted after <2022-01-21>. -->
+<!-- Before deletion, see: https://docs.gitlab.com/ee/development/documentation/#move-or-rename-a-page -->