Add sitemap and fully automate article creation

Article pages now insert the cover images automatically & an excerpt is used instead of a description for the page description.

_layouts/article.html

@@ -15,6 +15,14 @@
             <main class="main-content" id="main-content">
                 <div class="mdc-top-app-bar--short-fixed-adjust">
                     <h2 class="mdc-typography--headline2">{{ page.title }}</h2>
+                    <h3 class="mdc-typography--subtitle1">
+                        <time class="timeago" datetime="{{ page.date | date: '%Y-%m-%d' }}T{{ page.date | date: '%H:%M:%S' }}">{{ page.date | date: '%A %e %B %Y' }}</time>
+                         | by {{ page.authors }}
+                    </h3>
+                    <figure>
+                        <img src="/images/{{ page.url | relative_url | remove: '.html' | replace: '/', '-' | remove: '-articles-' }}--cover.jpg" style="width:100%;">
+                        <figcaption class="mdc-typography--caption-text">{{ page.image-caption }}</figcaption>
+                    </figure>
                     {{ content }}
                 </div>
             </main>