Configuration for upstream review apps

Contributors can deploy review apps of the Docs website from our upstream projects (gitlab, gitlab-runner, etc).

We need to figure out:

• How might this change if we're going to want to run a Hugo build in all docs pipelines anyways (to validate shortcodes)?
• What config do we need in this project to support that functionality?
• What will we need to update in the upstream projects, if anything?
• Will we need docs changes?

See https://docs.gitlab.com/ee/development/documentation/review_apps.html