Add UpCloud sponsorship blurb

Author: Kleidukos

site/index.html

@@ -556,6 +556,9 @@ <h2>Sponsors</h2>
          <div class=" span6 col-sm-6">
             <p><strong><a href="https://www.digitalocean.com/">Digital Ocean</a></strong> hosts critical Haskell.org infrastructure.</p>
          </div>
+         <div class=" span6 col-sm-6">
+            <p><strong><a href="https://upcloud.com/">UpCloud</a></strong> provides high-performance cloud infrastructure that powers GHC's CI pipelines.</p>
+         </div>
       </div>
    </div>
 </div>