Add GitHub Pages for archinstall[1]

I don't expect more GitHub Pages to be set up, so using a explict
resource instead of abstracting it, is fine I think.

[1] https://github.com/archlinux/archinstall/issues/2098

roles/prometheus/defaults/main.yml

@@ -28,6 +28,7 @@ blackbox_targets:
     - https://alpm-types.archlinux.page/alpm_types/
     - https://america.archive.pkgbuild.com
     - https://america.mirror.pkgbuild.com
+    - https://archinstall.archlinux.page
     - https://archive.archlinux.org
     - https://archlinux.org
     - https://archlinux.page

tf-stage1/archlinux.tf

@@ -450,6 +450,13 @@ resource "hetznerdns_record" "archlinux_page_origin_mx" {
   type    = "MX"
 }
 
+resource "hetznerdns_record" "archlinux_page_archinstall_cname" {
+  zone_id = hetznerdns_zone.archlinux_page.id
+  name    = "archinstall"
+  value   = "archlinux.github.io."
+  type    = "CNAME"
+}
+
 resource "hetznerdns_record" "archlinux_page_origin_ns3" {
   zone_id = hetznerdns_zone.archlinux_page.id
   name    = "@"