Explain how to override output collection index page (#6424)

Merge pull request 6424

docs/_docs/collections.md

@@ -87,6 +87,20 @@ For example, if you have `_my_collection/some_subdir/some_doc.md`,
 it will be rendered using Liquid and the Markdown converter of your
 choice and written out to `<dest>/my_collection/some_subdir/some_doc.html`.
 
+If you wish a specific page to be shown when accessing `/my_collection/`,
+simply add `permalink: /my_collection/index.html` to a page.
+To list items from the collection, on that page or any other, you can use:
+
+{% raw %}
+```liquid
+{% for item in site.my_collection %}
+  <h2>{{ item.title }}</h2>
+  <p>{{ item.description }}</p>
+  <p><a href="{{ item.url }}">{{ item.title }}</a></p>
+{% endfor %}
+```
+{% endraw %}
+
 <div class="note info">
   <h5>Don't forget to add YAML for processing</h5>
   <p>