docs: add ADR on Course Authoring Feature Flag Implementation Details #524

Job	Run time
tests (ubuntu-latest, 3.11, docs)	43s
tests (ubuntu-latest, 3.11, django42)	54s
tests (ubuntu-latest, 3.11, quality)	48s
tests (ubuntu-latest, 3.11, django52)	53s
tests (ubuntu-latest, 3.12, quality)	49s
tests (ubuntu-latest, 3.12, django42)	59s
tests (ubuntu-latest, 3.12, pii_check)	32s
tests (ubuntu-latest, 3.12, docs)	51s
tests (ubuntu-latest, 3.12, django52)	55s
tests (ubuntu-latest, 3.11, pii_check)	32s
	7m 56s

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