WIP: Adds the first class vulns documentation

What does this MR do?

This adds a first draft of the first class vulnerabilities documentation.

It's probably far from accurate, but let's get a discussion going.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• [-] Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Closes #204847 (closed)