Use site.docs instead of site.pages.

site/_config.yml

@@ -7,3 +7,7 @@ excerpt_separator: noifniof3nioaniof3nioafafinoafnoif
 repository: https://github.com/jekyll/jekyll
 help_url: https://github.com/jekyll/jekyll-help
 google_analytics_id: UA-50755011-1
+
+collections:
+  docs:
+    output: true

site/_includes/docs_option.html

@@ -3,7 +3,7 @@
 {% for item in items %}
   {% assign item_url = item | prepend:"/docs/" | append:"/" %}
 
-  {% for p in site.pages %}
+  {% for p in site.docs %}
     {% if p.url == item_url %}
       <option value="{{ site.url }}{{ p.url }}">{{ p.title }}</option>
     {% endif %}

site/_includes/docs_ul.html

@@ -10,7 +10,7 @@
     {% assign c = "" %}
   {% endif %}
 
-  {% for p in site.pages %}
+  {% for p in site.docs %}
     {% if p.url == item_url %}
       <li class="{{ c }}"><a href="{{ site.url }}{{ p.url }}">{{ p.title }}</a></li>
     {% endif %}