Move external URL to step to start

The external URL really should already be set on any real GitLab
instance before they get to configuring Gitaly HA. Moving this to an
early step helps prevent it being muddled into the actual HA setup.

doc/administration/gitaly/praefect.md

@@ -551,6 +551,16 @@ Particular attention should be shown to:
    sudo -i
    ```
 
+1. Configure the `external_url` so that files could be served by GitLab
+   by proper endpoint access by editing `/etc/gitlab/gitlab.rb`:
+
+   You will need to replace `GITLAB_SERVER_URL` with the real external facing
+   URL on which current GitLab instance is serving:
+
+   ```ruby
+   external_url 'GITLAB_SERVER_URL'
+   ```
+
 1. Add the Praefect cluster as a storage location by editing
    `/etc/gitlab/gitlab.rb`.
 
@@ -590,16 +600,6 @@ Particular attention should be shown to:
    gitlab_shell['secret_token'] = 'GITLAB_SHELL_SECRET_TOKEN'
    ```
 
-1. Configure the `external_url` so that files could be served by GitLab
-   by proper endpoint access by editing `/etc/gitlab/gitlab.rb`:
-
-   You will need to replace `GITLAB_SERVER_URL` with the real external facing URL on which
-   current GitLab instance is serving:
-
-   ```ruby
-   external_url 'GITLAB_SERVER_URL'
-   ```
-
 1. Add Prometheus monitoring settings by editing `/etc/gitlab/gitlab.rb`.
 
    You will need to replace: