Drupal - How do I show the site slogan?

After some desperate digging I found the solution (come on Drupal guys, don't be shy, give us some documentation!): Go to /admin/structure/block and click the 'configure' button of the block named 'Site branding'. There you can choose which parts of site branding are shown.

This is how to get the site slogan to pring in your theme page template without using the branding block (bootstrap or not):

function THEME_preprocess_page(&$vars) {

    $site_config = \Drupal::config('system.site');

    $vars['site_name'] = $site_config->get('name');
    $vars['site_slogan'] = $site_config->get('slogan');

}