Add PDF version of The Great Gatsby to test documents

Add a full novel as a test document for testing performance and
behaviors related to long PDF documents, such as re-anchoring when pages
are rendered and un-rendered while scrolling through the document.

We try to keep example PDFs small to avoid bloating the git repository, but
this one is on the larger side (~700KB) due to its length.

This PDF was produced by downloading the EPUB version of the novel [1]
from Project Gutenberg and then converting it to a PDF using Calibre [2]

[1] https://www.gutenberg.org/ebooks/64317
[2] https://calibre-ebook.com

dev-server/templates/index.mustache

@@ -31,6 +31,7 @@
     <li><a href="/pdf/widdershins">Widdershins (Wikipedia)</a></li>
     <li><a href="/pdf/painting"><em>The Development of British Landscape Painting in Water-Colours</em></a> (excerpt)
       by Alexander J. Finberg & E.A. Taylor (this PDF does not have select-able text)</li>
+    <li><a href="/pdf/gatsby">The Great Gatsby</a> (full novel)</li>
   </ul>
 
   <h2>Page feature tests</h2>