Datetime format (#2844)

* datetime_format

* page__meta

* page__date

* page__date test

* update docs

* update docs

_includes/page__meta.html

@@ -5,7 +5,8 @@
       {% assign date = document.date %}
       <span class="page__meta-date">
         <i class="far {% if include.type == 'grid' and document.read_time and document.show_date %}fa-fw {% endif %}fa-calendar-alt" aria-hidden="true"></i>
-        <time datetime="{{ date | date_to_xmlschema }}">{{ date | date: "%B %-d, %Y" }}</time>
+        {% assign date_format = site.date_format | default: "%B %-d, %Y" %}
+        <time datetime="{{ date | date_to_xmlschema }}">{{ date | date: date_format }}</time>
       </span>
     {% endif %}