Document "Force domains with SSL certificates to use HTTPS" Pages setting

The following discussion from gitlab-com/www-gitlab-com!10896 (merged) should be addressed:

@marcia started a discussion: (+2 comments)

@bikebilly we need to update the docs and link them from here. From what I understood, this is a new setting available for GitLab admins, so this is the doc to update: https://docs.gitlab.com/ce/administration/pages/

Edited May 07, 2018 by Achilleas Pipinellis