Add initial version of email ingestion blueprint

What does this MR do and why?

Meta: Replace mail_room email ingestion with sc... (#393157) was used to define a first draft version of a proposal to move email ingestion from the separate mail_room process right into the GitLab application using scheduled Sidekiq jobs. This is the MR that adds a first iteration of the Architecture Evolution Blueprint for the Architecture Evolution Workflow.

Review app: You can also use this link to view the diagrams and the final layout:

https://main-ee-122659.docs.gitlab-review.app/ee/architecture/blueprints/email_ingestion/index.html