Viewing HTML GitLab Artifacts requires GitLab Pages

What does this MR do?

This clarifies that the new 10.1 feature supporting viewing HTML Artifacts requires GitLab Pages to be enabled.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

I was trying to view HTML Artifacts within our internal GitLab instance and could not get this to work.

I tested this on GitLab.com and realized that this feature requires GitLab Pages, which currently isn't enabled on our internal GitLab instance.

This wasn't clear reading the documentation which just indicates that it is now available in GitLab 10.1, but makes sense that user defined HTML artifacts should not be loading on the base GitLab domain to prevent malicious artifacts from loading and possibly stealing credentials.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/3864

~"docs-fix"