docs: add ADR on Course Authoring Migration Process Details

[rodmgwgu]

This PR adds an ADR specifying the implementation strategy for the Migration Process for Course Authoring AuthZ.

Related issue: #184

Merge checklist:
Check off if complete or not applicable:

• Version bumped
• Changelog record added
• Documentation updated (not only docstrings)
• Fixup commits are squashed away
• Unit tests added/updated
• Manual testing instructions provided
• Noted any: Concerns, dependencies, migration issues, deadlines, tickets

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[dwong2708]

Just pointing out the case where a legacy role doesn’t have an equivalent in the new system. When the flag is enabled, would it become orphaned?

[dwong2708]

On a closer read, it seems this scenario won’t occur because all legacy roles will exist in the new system, while not all new-system roles exist in the legacy system. Is that correct?

[rodmgwgu]

Correct, I think all legacy roles should have equivalents on the new system, I think that's easy to achieve.