Fix up audit docs

The audit docs page could do with a lot of love.

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

Some specific thoughts:

1. Screenshots above are super old
2. There are a lot more events that are recorded than stated, for example, the "security events" table.
3. Audit Log section is not immediately obvious it's EEP only
4. I'm considering changing the name of Audit Log to "Audit Center" or something like that, this way it makes the distinction between "Audit events" & "Audit log" (should be Centralised Audit Log) less ambiguous. wdyt @williamchia

@axil are you able to help?

@fjsanpedro perhaps you can help with the list of events we log? this should include user authentication, group & project events