index: remove PDF and ePub versions

prawn-svg cannot render SVG fropm mermaid, see
mermaid-js/mermaid#2485 (comment)

For more info.

Author: willcl-ark

index.html

@@ -33,8 +33,7 @@
   <div class="dah_card">
     <p><b>Get onboarded</b></p>
     <p>Read the <a href="bin/book.html">entire onboarding guide</a> in a
-    convenient single-page format.  Also available in <a
-    href="404">ePub</a> and <a href="404">PDF</a>.</p>
+    convenient single-page format.</p>
   </div>
 
   <div class="dah_card">