Adds email ingestion design doc and readds danger (with design docs rule)

Why is this change being made?

Contributes to Move Architecture Blueprints to Public Handbook (#279 - closed)

This adds the first design doc to the handbook. We'll use this MR to test the danger integration for the architecture evolution workflow and get a feel for how design docs behave in the handbook.

Also "readds" danger to provide the same danger review functionality we currently have in the gitlab project for the architecture evolution design docs.

To enable this integration danger-review is re-added with only two danger rules:

• architecture
• z_retry_link

Reviewer roulette is kept disabled. See #238

Necessary adjustments to existing design docs

Below I documented all the adjustments I needed to make to get this doc from the old format to the new format supported by hugo:

1. Always add a design document to a subfolder. The main file should be called _index.md. If you use a different format the document won't appear in the list of documents.
2. Add a title frontmatter item with your title.
3. Remove the headline one # Title from the document
4. Add toc_hide: true to front matter so we don't display every design doc in the left sidebar navigation.
5. Add the {{< design-document-header >}} shortcode before the first content of your design document, so we'll display the design documents header properly.
6. Replace all links to the documentation with absolute links.

Author and Reviewer Checklist

Please verify the check list and ensure to tick them off before the MR is merged.

• Provided a concise title for this Merge Request (MR)
• Added a description to this MR explaining the reasons for the proposed change, per say why, not just what
  • Copy/paste the Slack conversation to document it for later, or upload screenshots. Verify that no confidential data is added, and the content is SAFE
• Assign reviewers for this MR to the correct Directly Responsible Individual/s (DRI)
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the Maintained by section on the page being edited
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies
  • The when to get approval handbook section explains the workflow in more detail
• For transparency, share this MR with the audience that will be impacted.
  • Team: For changes that affect your direct team, share in your group Slack channel
  • Department: If the update affects your department, share the MR in your department Slack channel
  • Division: If the update affects your division, share the MR in your division Slack channel
  • Company: If the update affects all (or the majority of) GitLab team members, post an update in #whats-happening-at-gitlab linking to this MR
    • For high-priority company-wide announcements work with the internal communications team to post the update in #company-fyi and align on a plan to circulate in additional channels like the "While You Were Iterating" Newsletter

Commits

• Adds email ingestion design doc

This adds the first design doc to the handbook. We'll use this as a starting point to add more docs.

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Screenshots

!6869 (comment 1979038510)