Docs: Update CI/CD intro - workflow illustration

What does this MR do?

• The second iteration of CI/CD workflow example illustration: https://docs.gitlab.com/ee/ci/introduction/index.html#basic-cicd-workflow

Related issues

• Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/59108
• For &784 (closed)
• Follow-up from https://gitlab.com/gitlab-org/gitlab-ce/issues/58720 // https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/25934 and https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/25611

Author's checklist

• Follow the Documentation Guidelines and Style Guide.
• Apply the ~Documentation label.

Review checklist

All reviewers can help ensure accuracy, clarity, completeness, and adherence to the Documentation Guidelines and Style Guide.

1. Primary Reviewer

• Review by a code reviewer or other selected colleague to confirm accuracy, clarity, and completeness. This can be skipped for minor fixes without substantive content changes.

2. Technical Writer

• Optional: Technical writer review. If not requested for this MR, must be scheduled post-merge. To request for this MR, assign the writer listed for the applicable DevOps stage.

3. Maintainer

1. Review by assigned maintainer, who can always request/require the above reviews. Maintainer's review can occur before or after a technical writer review.
2. Ensure a release milestone is set and that you merge the equivalent EE MR before the CE MR if both exist.
3. If there has not been a technical writer review, create an issue for one using the Doc Review template.