Docs/UX/Design: New diagram to illustrate GitLab CI/CD

Replace old graph with new (and simpler) illustration:

• Old graph (deleted):

• New graph for https://docs.gitlab.com/ee/ci/introduction/index.html#basic-cicd-workflow (MVC) to replace the old graph:

Original proposal	Current version

References

Follow up from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/25611

MR with this graph: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/25934

REVIEW APP: http://review-docs-docs-cgqw62.178.62.207.141.xip.io/ce/ci/introduction/index.html#basic-cicd-workflow