Adds note to architecture design docs page about move to handbook

What does this MR do and why?

Contributes to Move Architecture Blueprints to Public Handbook (gitlab-com/content-sites/handbook#279)

We're about to move architecture design documents from the Gitlab project (docs page) over to the public handbook.

This MR adds a note that this move will happen to the current architecture design docs page and the design docs template.

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