shortcode: Add Google Docs

Enables easily embedding Google Slides.

To use, copy and paste the share link from Google Docs in the form:
`{{< gdocs "https://..." >}}`

layouts/partials/css/academic.css

@@ -238,6 +238,10 @@ small,
   font-size: .75em;
 }
 
+.responsive-wrap iframe {
+  max-width: 100%;
+}
+
 /*************************************************
  *  Modals.
  **************************************************/

layouts/shortcodes/gdocs.html

@@ -0,0 +1,3 @@
+<div class="responsive-wrap">
+  <iframe src="{{ strings.TrimSuffix (index (last 1 (split (.Get 0) "/")) 0) (.Get 0) }}embed?start=false&loop=false&delayms=3000" frameborder="0" width="960" height="569" allowfullscreen="true" mozallowfullscreen="true" webkitallowfullscreen="true"></iframe>
+</div>