_includes/header.html

<nav class="navbar navbar-expand-lg navbar-light bg-light sticky-top shadow-lg">
    <a class="navbar-brand" href="#">Navbar</a>
    <button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarSupportedContent" aria-controls="navbarSupportedContent" aria-expanded="false" aria-label="Toggle navigation">
        <span class="navbar-toggler-icon"></span>
    </button>

    <div class="collapse navbar-collapse" id="navbarSupportedContent">
        <ul class="navbar-nav mr-auto" style="margin-left: 25%">
            <li class="nav-item active">
                <a class="nav-link" href="index.html">Home <span class="sr-only">(current)</span></a>
            </li>
            <li class="nav-item">
                <a class="nav-link" href="about.html">About</a>
            </li>
            <li class="nav-item">
                <a class="nav-link" href="documentation.html">Documentation</a>
            </li>
            <li class="nav-item">
                <a class="nav-link" href="publications.html">Publications</a>
            </li>
            <li class="nav-item dropdown">
                <a class="nav-link dropdown-toggle" data-toggle="dropdown" href="#" role="button" aria-haspopup="true" aria-expanded="false">Apache</a>
                <div class="dropdown-menu">
                    <a class="dropdown-item" href="http://www.apache.org/foundation/how-it-works.html">Apache Software Foundation</a>
                    <a class="dropdown-item" href="http://www.apache.org/licenses/">Apache License</a>
                    <a class="dropdown-item" href="http://www.apache.org/foundation/sponsorship.html">Sponsorship</a>
                    <a class="dropdown-item" href="http://www.apache.org/foundation/thanks.html">Thanks</a>
                </div>
            </li>
            </li>
        </ul>
    </div>
</nav>