chore: explicitly mention AuthKeys url

infra.tf

 # dummy key will be used to avoid servers root passwords being mailed
-# will use AK to setup real authorization
+# will use https://authkeys.io to setup real authorization
 resource "hcloud_ssh_key" "dummy-key" {
   name       = "dummy-key"
   public_key = "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIDG6KCXcL6bgJKD/ZbzLDeZms6TJwUz7K0yf+KCvfjxV dummy-key"