Adds redirect for email ingestion design docs

What does this MR do and why?

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

We're about to move architecture design documents from the docs page (md files in gitlab project) to the public handbook. We already moved the design doc for email ingestion, so this is the MR to test how redirects work on the docs page.

Adds a redirect from the old email ingestion design doc (docs site) https://docs.gitlab.com/ee/architecture/blueprints/email_ingestion/ to the new home of architecture design docs in the public handbook at https://handbook.gitlab.com/handbook/engineering/architecture/design-documents/email-ingestion/

❗ FYI the "old" content still exists. We'll only remove the old content once the redirect is in place.

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