Add docs for Audit Events in Free tier

What does this MR do?

This expands the AuditEvent documentation to explain the single type of Audit Event that is created in the Free tier, how it can be viewed, and how it is relevant after upgrading to a paid tier. This was prompted by multiple inquiries from TAMs/customers.

Background

In Slack#g_manage_compliance on 2021-05-05, @tancnle wrote:

We only audit successful logins in CE atm. I think the reason why it is not in the document is that the users won't be able to access the Audit Event search page on Project/Group/Admin on CE (it is a Premium feature). The users can see those successful login audit events under https://gitlab.com/profile/audit_log (we need to document this page) (edited)

So to answer the customer question on how much back-access they’ll have to old audit events, I would say it is minimal, only successful login events under Admin Audit Events.

For reference, we discuss about removing the feature exclusion when writing audit events, so that the users might get more out of the upgrade.

Note: We decided NOT to remove the feature exclusion, meaning successful login events will continue to be the only Audit Events captured in the Free tier.

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team