Add section description to security training configuration

What does this MR do and why?

In this MR a section description is added to the "Security training" section.

This MR is part of the new security training feature, please see the epic for details and the implementation breakdown > &6176 (closed)

Screenshots or screen recordings

How to set up and validate locally

This feature is behind a feature flag, to enable

rails c

Feature.enable(:secure_vulnerability_training)

1. Navigate to Security & Compliance > Configuration
2. Click on the "Vulnerability Management" tab
3. Under the "Security training" section, you will see the description

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #346488 (closed)