Link to GitHub history of page

The previous commit added the last updated date to the bottom of each
contentful page. This adds a link to the respective GitHub history page,
for more detailed information (e.g. to check that the last update was
meaningful and not just a dev fixing formatting across multiple files)

The last updated date could in some cases give a misleading sense of
security that the page is more up to date than its content, so this
provides a quick way to check

src/_includes/page-content.html

@@ -1,5 +1,6 @@
 {% capture last_modified_at %}{{ page.last_modified_at | date: '%s' }}{% endcapture %}
 {% capture last_reviewed_at %}{{ page.last_reviewed_at | date: '%s' }}{% endcapture %}
+{% capture commit_history_url %}https://github.com/dxw/playbook/commits/main/src/{{ page.path }}{% endcapture %}
 
 <article class="page-content">
   <div>
@@ -21,7 +22,7 @@ <h1>{{ page.title }}</h1>
             Last reviewed: {{ page.last_reviewed_at | date: "%-d %B %Y" }}
             <br>
           {% endif %}
-          Last updated: {{ page.last_modified_at | date: "%-d %B %Y" }}
+          Last updated: {{ page.last_modified_at | date: "%-d %B %Y" }} (<a href="{{ commit_history_url }}">history</a>)
         </i>
       </p>    
     {% endif %}