Engagement - Event Tracking

Placeholder for event tracking documentation. Unlike other parts of the Help Center, we will continue to keep the [DEPRECATED] Event Tracking content updated in the FZ KB until the Help Center is ready to publish, at which point we will take a snapshot of the content from the FZ KB to the Help Center. This is to prevent double-entry during the transition, since the event tracking documentation gets updated frequently as we update existing events or introduce new ones.