Offline Direct Transfer migrations architecture design document

Why is this change being made?

This MR adds an architecture design document for offline Direct Transfer migrations. It's not entirely complete, but this is the basis of what offline imports could look like, and in the future, how customers can import from anywhere. Some specific sections that will need further iteration include:

• Exported file structure and metadata file structure will need to be outlined in detail.
• Import-side changes will also need to be outlined in more to describe how all non-file-based pipelines file-based pipelines can import file-based imports when there's no connection to the source.
• Specify the earliest version of GitLab that can be exported with Congregate.
• Export-side changes necessary to support an API and UI within GitLab itself for new enough source versions.

An architecture design document seemed warranted because this involves significant changes to Direct Transfer, it requires creating a standard format for files imported by Direct Transfer, and involves changes to Congregate that support offline migrations.

References

• Discovery issue: Discovery - Support automated migrations betwee... (gitlab-org/gitlab#517490 - closed)
• Offline migrations epic: Support semi-automated migrations between offli... (gitlab-org&8985)
• Congregate documentation
• Architecture design workflow handbook page

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Commits

• Added overview of offline migrations in new architecture design document

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