Docs/diagram for `pages:deploy` flow

Summary

I can't find the diagram of the workflow between GitLab and gitlab-pages server that shows in details how pages:deploy process works.

I need one to see how to solve gitlab#386141 which is about adding a job page with description of error if an error occurs.

Steps to reproduce

1. Go to development docs and see there is no description of the process, from start of the pipeline, through deploy, to serving https://docs.gitlab.com/ee/development/pages/

Possible fixes

Place a diagram like used in #377 (closed) to describe the whole process of deploying and serving pages.