Add a placeholder preview for the navigation toolbar

The idea is great and the code in the MR looks sensible to me.

However, I have not been able to find a way to make it work. This is probably out of my limited FE knowledge.

Adding some debugging information that could be useful for a FE development who could take a look into this:

Replace code in NavigationRenderer::buildNavigation() by something similar to this:

  public function buildNavigation(array &$page_top): void {
    $page_top['navigation'] = [
        '#type' => 'html_tag',
        '#tag' => 'aside',
        '#attributes' => [
          'class' => ['admin-toolbar'],
          'id' => 'admin-toolbar',
        ],
        'child' => [
          '#type' => 'container',
          '#attributes' => [
            'class' => ['admin-toolbar__displace-placeholder'],
          ],
        ],
    ];
  }

When any page is loaded, the space for the navigation bar should be reserved, but empty.

Once that behavior is achieved, that chunk of HTML should be moved to the '#lazy_builder_preview' render array property in the original output.