The site logo moved to a block in drupal 8 but with this snippet you are still able to get the custom logo path, as defined in your settings file.
/**
* Implements hook_preprocess_page().
*/
function MYTHEME_preprocess_page(&$variables) {
// Theme settings
$variables['logopath'] = '/' . \Drupal::config('MYTHEME.settings')->get('logo.path');
}
Now you can use
<img src="{{ logopath }}" />
in your page.html.twig