Add the section of introduction to the paper book (#1114)

* Use Roboto mono as the font of the footers

* Bug fixes

* Add the introduction to the paper book.

* Add the 'new' label to the toc

* Turn off the autoplay

* Polish the intro to paperbook

overrides/stylesheets/extra.css

@@ -522,3 +522,17 @@ a:hover .text-button span {
     flex-basis: 25%;
   }
 }
+
+.video-container {
+  position: relative;
+  padding-bottom: 56.25%; /* 16:9 */
+  height: 0;
+}
+
+.video-container iframe {
+  position: absolute;
+  top: 0;
+  left: 0;
+  width: 100%;
+  height: 100%;
+}