Docs: illustration for CI/CD intro guide (CI, CD, CD)

Type of issue

Improving an existing doc.

Problem to solve

1. Add illustrations for each continuous method:

• Continuous Integration: https://docs.gitlab.com/ee/ci/introduction/index.html#continuous-integration
• Continuous Delivery: https://docs.gitlab.com/ee/ci/introduction/index.html#continuous-delivery
• Continuous Deployment: https://docs.gitlab.com/ee/ci/introduction/index.html#continuous-deployment

2. Add illustration for DevOps stages (from https://gitlab.com/gitlab-org/gitlab-ce/issues/59108#note_152357469) => https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/27278
3. Add illustration to ci/README.md (overview GitLab CI/CD)

Further details

Illustrate each continuous method as we did here: https://about.gitlab.com/2016/08/05/continuous-integration-delivery-and-deployment-with-gitlab/

Proposal

• Create 3 diagrams, one for each method (CI/CD/CD).
• Create illustration for DevOps stages (Verify, Package, Release) => https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/27278
• Create illustration for CI/CD overview in ci/README.md

Who can address the issue

@marcia @jlenny @mikelewis

Other links/references

• Follows https://gitlab.com/gitlab-org/gitlab-ce/issues/59108
• Follow-up from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/25611 and https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/25934