Document duo_related flag on member_destroyed audit events

What does this MR do?

Documents the duo_related flag added to member_destroyed audit events in !239648 (merged). When a Duo Agent Platform service account membership is removed, the audit event carries duo_related: true in its details object, so SIEM tools can filter on the event detail instead of service account naming patterns. This adds a section to the audit event schema page describing the field.

Stacked on !239648 (merged) and targets its branch, so this MR shows only the documentation commit. It retargets to master when that MR merges.

Related issues

Related to https://gitlab.com/gitlab-org/gitlab/-/issues/593024

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