Update docs/configuring-playbook-matrix-ldap-registration-proxy.md: add the common section "Troubleshooting"

Signed-off-by: Suguru Hirahara <acioustick@noreply.codeberg.org>

docs/configuring-playbook-matrix-ldap-registration-proxy.md

@@ -52,3 +52,7 @@ ansible-playbook -i inventory/hosts setup.yml --tags=setup-all,start
 The shortcut commands with the [`just` program](just.md) are also available: `just install-all` or `just setup-all`
 
 `just install-all` is useful for maintaining your setup quickly ([2x-5x faster](../CHANGELOG.md#2x-5x-performance-improvements-in-playbook-runtime) than `just setup-all`) when its components remain unchanged. If you adjust your `vars.yml` to remove other components, you'd need to run `just setup-all`, or these components will still remain installed. Note these shortcuts run the `ensure-matrix-users-created` tag too.
+
+## Troubleshooting
+
+As with all other services, you can find the logs in [systemd-journald](https://www.freedesktop.org/software/systemd/man/systemd-journald.service.html) by logging in to the server with SSH and running `journalctl -fu matrix-ldap-registration-proxy`.