causeway-standard-theme/causeway/components/site-menu.html

<nav class="navbar site-menu" role="navigation">
  <ul class="nav navbar-nav">
    <li>
      <a href="docs/">
        Documents
      </a>
    </li>
    <li class="dropdown">
      <a href="#"  data-hover="dropdown">
        Archive
        <span class="fa fa-chevron-down"></span>
      </a>
      <ul class="dropdown-menu">
        <li>
          <a href="archive/v-1/" target="_blank">
            Version 1
          </a>
        </li>
        <li>
          <a href="archive/v-2/" target="_blank">
            Version 2
          </a>
        </li>
      </ul>
    </li>
  </ul>
</nav>