docs: function generateHTML(fileName) {
    return new Promise( resolve => {
      const location = fileName === 'index.html' ? '/' : `/${fileName}`;
      match({routes, location}, (error, redirectLocation, renderProps) => {
        let html = ReactDOMServer.renderToString(
          <RouterContext {...renderProps} />
        );
        html = '<!doctype html>' + html;
        let write = fsp.writeFile(path.join(docsBuilt, fileName), html);
        resolve(write);
      });
    });
  }