Add help link for opt in JWT setting

What does this MR do and why?

As titled. Setting should now have a Learn more link that leads to the relevant docs.

Screenshots or screen recordings

How to set up and validate locally

1. Go to Settings > CI/CD > Token Access > Limit JSON Web Token (JWT) access.
2. Description should now have a Learn more link.

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.