404 page on docs site requires sign-in

Description

A non-existent page on the docs.gitlab.com site should not require a user to log in.

Steps to reproduce

1. Log out of GitLab.com
2. Visit https://docs.gitlab.com/ee/ci/pipelines/cicd_minutes.html when not logged in

Actual result

A log in page appears. When you log in, you are redirected to a 404.

Expected result

No log in page. Just show the 404.

Further details

• This also happens on the Handbook site (404 example) and the design.gitlab.com (404 example) site.
• It does not happen on other sites in the same gitlab-org/technical-writing group (like this test site -- 404 example).
  • So it's probably related to the *.gitlab.com domains (although it does not happen on about.gitlab.com). Requested infra help here: gitlab-com/gl-infra/production-engineering#26907 (closed)

Originally reported by @menaechmi in gitlab#547708 (closed).