docs: add ADR for course authoring automatic migration #750

Job	Run time
tests (ubuntu-latest, 3.12, django52)	1m 41s
tests (ubuntu-latest, 3.12, pii_check)	31s
tests (ubuntu-latest, 3.12, quality)	1m 2s
tests (ubuntu-latest, 3.12, docs)	45s
	3m 59s

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