Add key missing components to reference architecture

Certain key features of GitLab are not included in reference architecture that I believe should be:

• Container registry gitlab-org/gitlab#468094
• Pages
• Reply by Email
• Shared runner
• Advanced search
• GitLab agent service? may be working out of box

Debatable:

• Kroki

Recommend promoting this to epic.

See also https://docs.gitlab.com/ee/development/architecture.html