Docs: CI/CD intro guide illustration (2nd iteration)

Type of issue

Improving an existing doc.

Problem to solve

Improve CI/CD illustration: https://docs.gitlab.com/ee/ci/introduction/index.html#basic-cicd-workflow:

Further details

Improve current illustration based on PM's feedback.

Proposal

To be presented by the PMs.

Who can address the issue

@jlenny @brendan @marcia

Other links/references

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