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

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

docs/configuring-playbook-matrix-registration.md

@@ -101,3 +101,7 @@ ansible-playbook -i inventory/hosts setup.yml \
 ```
 
 The shortcut command with `just` program is also available: `just run-tags list-matrix-registration-tokens`
+
+## 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-registration`.