Site: Filter through just the docs collection (#8170)

Merge pull request 8170

docs/_includes/docs_contents_mobile.html

@@ -4,7 +4,7 @@
     {% for section in site.data.docs_nav -%}
     <optgroup label="{{ section.title }}">
     {%- for item in section.docs -%}
-      {% assign p = site.documents | where: "url", item.link | first %}
+      {% assign p = site.docs | where: "url", item.link | first %}
       <option value="{{ p.url | relative_url }}">
         {{- p.menu_name | default: p.title -}}
       </option>