GitLab Pages supports projects hosted under a subgroup, but not subgroup
websites. That means that only the highest-level group supports

i.e.:
You created a group for your engineering department called
`engineering`, a subgroup for all your documentation websites called
`docs`,and a project within this subgroup is called `workflows`.
Your project URL is `https://gitlab.com/engineering/docs/workflows/`.
Once you enable GitLab Pages for this project, the site will live under
`https://engineering.gitlab.io/docs/workflows`.