Update audit events docs

https://docs.gitlab.com/ee/administration/audit_events.html

• Update images
• Add Overview and Use cases sections

changed the description

mentioned in issue #2919 (closed)

mentioned in issue #2703 (closed)

marked the checklist item Add Overview and Use cases sections as completed

Doc seems to have a missing link on the bottom: http://docs.gitlab.com/ee/administration/audit_events.html#audit-log-admin-only

changed milestone to %9.5

I can work on this after the 25th.

mentioned in issue gitlab-com/marketing#1245 (closed)

@jvasseur