More info in Expression Reference about event types

### Current Behavior

When opening the Expression Reference, i get some info:

### Proposed Behavior

A clickable link to notifications schema to know what kind of information is possible

https://dependencytrack.github.io/docs/next/reference/notifications/schema/

### Checklist

- [x] I have read and understand the [contributing guidelines](https://github.com/DependencyTrack/dependency-track/blob/main/CONTRIBUTING.md#filing-issues)
- [x] I have checked the [existing issues](https://github.com/DependencyTrack/dependency-track/issues) for whether this enhancement was already requested

Start by locating the Expression Reference view and reading how its current event information is presented. Add a clickable link to the notifications schema shown in the issue, and confirm that the link is visible and opens the referenced schema.

Written by the indexing model from the issue text.