Create the module help page on Drupal 8

Here an exemple : Module name : mymodule

function mymodule_help($route_name, RouteMatchInterface $route_match) {

  if ($route_name == 'help.page.mymodule') {

    $current_url = Url::fromRoute('<current>');

    if (strstr($current_url->toString(), '/admin/help/simple_analyse') === FALSE) {

      // Because system_modules() executes hook_help() for each module to 'test'

      // if they will return anything, but not actually display it, we want to

      // return a TRUE value if this is not actually the help page.

      return TRUE;

    }

    $output = '<dl>';

    $output .= '<dt>' . ('List of the currently available tokens on this site') . '</dt>';

    $token_tree = array(

      '#theme' => 'token_tree',

      '#token_types' => 'all',

      '#click_insert' => FALSE,

      '#show_restricted' => TRUE,

    );

    /** @var \Drupal\Core\Render\RendererInterface $renderer */

    $renderer = \Drupal::service('renderer');

    $output .= '<dd>' . $renderer->render($token_tree) . '</dd>';

    $output .= '</dl>';

    return $output;

  }

}