website/_includes/nav.html

<div class="nav">
    <h1>{{ site.name }}</h1>
    <h2>{{ site.description }}</h2>
    <ul>
        {% for item in site.data.navigation %}
        <li>
            <a href="{{ item.url }}">{{ item.title }}</a>
        </li>
        {% endfor %}
    </ul>
</div>