[Documentation] Add instance scope for audit events in documentation

In !123882 (merged), we have introduced a new scope/entity for audit events 'InstanceScope' for instance level audit events.

Identified documentation locations for updating a new scope for audit events:

1. https://docs.gitlab.com/ee/development/audit_event_guide/#audit-event-schemas.
2. https://docs.gitlab.com/ee/administration/audit_events.html#export-to-csv
3. https://docs.gitlab.com/ee/administration/audit_events.html#view-audit-events
4. https://docs.gitlab.com/ee/api/audit_events.html#retrieve-all-instance-audit-events