[AWS monitoring]: Add CloudTrail use case for Elastic Agent

### Description

After the AWS monitoring section is refactored (#1672), we should add a new section on how to collect CloudTrail logs using the Elastic Agent.

### Resources

Some draft content is available in https://github.com/zmoog/public-notes/issues/79

### Which documentation set does this change impact?

Stateful and Serverless

### Feature differences

None?

### What release is this request related to?

N/A

### Collaboration model

The documentation team

### Point of contact.

**Main contact:** @zmoog