Enable gitlab pages

Fixes #228 (closed) and supersedes !163 (closed). Enables easy resolving of #61 (closed).

The test page is online at https://test.archlinux.org/. Its project is here: https://gitlab.archlinux.org/svenstaro/pages-test/pages It uses the GitLab built-in Let's Encrypt integration.

Changes are already rolled out.

Upon inspection and merging, the reviewer must:

• Remove hetznerdns_record.gitlab_pages_test_* from archlinux.tf and run terraform apply
• Remove the project https://gitlab.archlinux.org/svenstaro/pages-test

docs/gitlab-pages.md

 # Using GitLab Pages
 
-In order use GitLab Pages with Arch Linux, you'll have to specifically request a custom subdomain
+In order to use GitLab Pages with Arch Linux, you'll have to specifically request a custom subdomain
 below `pkgbuild.com` or `archlinux.org` to be assigned. We don't allow random projects to use Pages
 because of legal and safety reasons (we don't want people to be able to trick others into thinking
 something hosted below one of our domains is official).