Add Traefik Dashboard warning

docs/configuring-playbook-traefik.md

@@ -35,6 +35,8 @@ devture_traefik_dashboard_basicauth_user: YOUR_USERNAME_HERE
 devture_traefik_dashboard_basicauth_password: YOUR_PASSWORD_HERE
 ```
 
+**WARNING**: enabling the dashboard on a hostname you use for something else (like `matrix_server_fqn_matrix` in the configuration above) may cause conflicts. Enabling the Traefik Dashboard makes Traefik capture all `/dashboard` and `/api` requests and forward them to itself. If any of the services hosted on the same hostname requires any of these 2 URL prefixes, you will experience problems. So far, we're not aware of any playbook services which occupy these endpoints and are likely to cause conflicts.
+
 ## Additional configuration
 
 Use the `devture_traefik_configuration_extension_yaml` variable provided by the Traefik Ansible role to override or inject additional settings, even when no dedicated variable exists.