Updated the UI details of how to add Secret Variables to a CI/CD pipeline

What does this MR do?

Updates documentation to reflect changes in the GitLab UI.

Are there points in the code the reviewer needs to double check?

Verify that the location of Secret Variables is the same for all users, and not just for me (on my plan, with my kind of project, whatever).

Why was this MR needed?

I search for 15 minutes for "Settings > Variables" and couldn't find it.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

No issue opened.