Follow-up "Document auth_redirect_uri in charts for GitLab Pages"

The following discussion from !7516 (merged) should be addressed:

• @mnielsen started a discussion: (+2 comments)

  Document auth_redirect_uri here: https://docs.gitlab.com/charts/charts/gitlab/gitlab-pages/index.html#pages-specific-settings

  Reference documentation: https://docs.gitlab.com/ee/administration/pages/#global-settings