docs: allow a taxonomy- or course-scoped CompetencyRuleProfile to be deleted with its taxonomy or course

[mgwozdz-unicon]

Jesper pointed out on #613 that pinning CompetencyRuleProfile.competency_taxonomy to PROTECT would block deleting a taxonomy just because a rule profile happens to be scoped to it, even if nothing actually depends on that profile. This amends ADR-0002 Decision 7 so a taxonomy- or course-scoped CompetencyRuleProfile is deleted along with its taxonomy or course instead of blocking the delete.

This is safe because a taxonomy or course is only ever hard-deleted once nothing beneath it needs protecting (Decision 7 already guarantees that), so there's nothing left for the profile's archive-only retirement to protect at that point either.

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