WordPress shortcodes: add_shortcode, attributes, and nested content

/**
 * Add a shortcode.
 */
add_shortcode('chair-picker-tool', [ self::class, 'shortcode' ]);

/**
 * Shortcode.
 *
 * @return string
 */
public static function shortcode() {

    return "<p>Test</p>";
}

/**
 * Shortcode for the form.
 *
 * @return string
 */
public function shortcode_show_form()
{
    ob_start();

    require plugin_dir_path( __FILE__ ) . 'partials/shortcode-display.php';

    return ob_get_clean();
}