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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[bmtcril]

Just one comment otherwise LGTM!

[bmtcril]

Do we have a list of new roles that can't be back-migrated that we can add here?

[rodmgwgu]

Added a list of roles that won't be handled by the migration process. Thanks!

[bmtcril]

LGTM