site/_includes/docs_option.html

{% assign items = include.items %}

{% for item in items %}
  {% assign item_url = item | prepend:"/docs/" | append:".html" %}

  {% for p in site.docs %}
    {% if p.url == item_url %}
      <option value="{{ site.url }}{{ p.url }}">{{ p.title }}</option>
    {% endif %}
  {% endfor %}
{% endfor %}