Update doc for enabling or disabling GitLab CI

What does this MR do?

Update docs for Enabling or disabling GitLab CI with screenshots from GitLab CE 8.16.

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

No.

Why was this MR needed?

The current docs are too old against current GitLab (8.16).

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• [n/a] API support added
• Tests
  • [n/a] Added for this feature/bug
  • All builds are passing
• [n/a] 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?

Closes #27656 (closed)