govarch-website/_includes/menu.html

{% assign menu = site.data.menu %}

{% if menu %}
  <nav class="site-navigation">
    <ul>
    {% for item in site.data.menu %}
      <li>
        {% if item.url contains 'http' %}
          <a href="{{ item.url }}">
            {{ item.title }}
          </a>
        {% else %}
          <a href="{{ item.url | prepend: site.baseurl }}">
            {{ item.title }}
          </a>
        {% endif %}
      </li>
    {% endfor %}
    </ul>
  </nav>
{% endif %}