Update audit event docs

What does this MR do and why?

Update audit event docs:

• Moves audit event schema and schema examples from seperate pages into one page
  • https://main-ee-140273.docs.gitlab-review.app/ee/administration/audit_event_schema.html
• Removes unneeded examples page
• Rename audit events to audit event reports
  • https://main-ee-140273.docs.gitlab-review.app/ee/administration/audit_event_reports.html
• Removes unneeded audit event reports page

In a follow up MR we will update the example and https://docs.gitlab.com/ee/api/audit_events.html to match the current schema

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