Add link to AuditEvent instrumentation

To help encourage engineers to add missing AuditEvents on their
own, this adds a link to the instrumentation documentation in the
"Missing events" section of the documentation.

doc/administration/audit_events.md

@@ -193,7 +193,7 @@ on adding these events into GitLab:
 Don't see the event you want in any of the epics linked above? You can use the **Audit Event
 Proposal** issue template to
 [create an issue](https://gitlab.com/gitlab-org/gitlab/-/issues/new?issuable_template=Audit%20Event%20Proposal)
-to request it.
+to request it, or you can [add it yourself](../development/audit_event_guide/).
 
 ### Disabled events