Add documentation link to CI/CD Variables Form

What does this MR do and why?

This documentation link points to documentation explaining how environment scopes work, which assist in the understanding of the special * character when used in environment scopes.

Changelog: added

Screenshots or screen recordings

How to set up and validate locally

1. Navigate to a group or project
2. Navigate to Settings > CI/CD
3. Add a variable

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.

For #231529 (closed)