support relative paths for eca documentation

That validation seems to be contained in the upstream component, i.e. in the bpmn.io javascript package, not in the module here. So, there is nothing we can do about that other than you could open an issue in the bpmn.io project on GitHub and ask them to change their code.

However, you could easily add the following line to your settings.php to dynamically define the configuration of that domain:

$settings['eca.settings']['documentation_domain'] = 'https://yourdomain/ecaguide';

Depending on how your deployment and setup is being done, this could be automated, just like you put the $settings['trusted_host_patterns'] into the settings.php as well.