bootstrap/docs/_includes/nav-main.html

<div class="navbar navbar-inverse navbar-fixed-top bs-docs-nav">
  <div class="container">
    <a href="/" class="navbar-brand">Bootstrap</a>
    <button class="navbar-toggle" type="button" data-toggle="collapse" data-target=".bs-navbar-collapse">
      <span class="icon-bar"></span>
      <span class="icon-bar"></span>
      <span class="icon-bar"></span>
    </button>
    <div class="nav-collapse collapse bs-navbar-collapse">
      <ul class="nav navbar-nav">
        <li{% if page.slug == "welcome" %} class="active"{% endif %}>
          <a href="/welcome">Overview</a>
        </li>
        <li{% if page.slug == "css" %} class="active"{% endif %}>
          <a href="/css">CSS</a>
        </li>
        <li{% if page.slug == "components" %} class="active"{% endif %}>
          <a href="/components">Components</a>
        </li>
        <li{% if page.slug == "js" %} class="active"{% endif %}>
          <a href="/javascript">JavaScript</a>
        </li>
        <li{% if page.slug == "customize" %} class="active"{% endif %}>
          <a href="/customize">Customize</a>
        </li>
      </ul>
    </div>
  </div>
</div>