Adds migration note to architecture design docs dangerfile
What does this MR do and why?
Contributes to Move Architecture Blueprints to Public Handbook (gitlab-com/content-sites/handbook#279 - closed)
This MR adds a note to the danger review for architecture design docs to notify everyone on the MR that we'll migrate design docs to the public handbook shortly. It also adds resources where authors can seek help and get updates.