audit_events.md: major refactoring 2

• audit_events.md: add missing initiator field
• audit_events.md: shorten boilerplate
• audit_events.md: drop_user, drop_role jsons
• audit_events.md: track formal requirements
• audit_events.md: join_instance, expel_instance
• audit_events.md: details about initiator

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• Part of #209 (closed)
• Follow-up for !361 (merged)
• See also https://git.picodata.io/picodata/picodata/picodata/-/issues/340
• Target branch: main
• Changes should be cherry-picked to 24.2: yes
• Staging: https://docs.binary.picodata.io/picodata/rosik_audit_events/