Include a link to Doxygen documentation on the web

index.html

@@ -336,6 +336,9 @@ <h2>Contacts and Resources</h2>
                 <li class="">
                     <a href="http://geeqie.org/cgi-bin/gitweb.cgi?p=geeqie.git;a=summary"><i class="fa fa-git-square"></i> main source repository</a>
                 </li>
+                <li class="">
+                    <a href="https://cclark.uk/geeqie/doxygen/html/index.html"><i class="fa fa-book"></i> developer documentation</a>
+                </li>
                 <div title="In order to minimize developer workload,&#013;please visit the mailing list archive&#013;and subscribe to the mailing list&#013;before making a post.&#013;&#013;You will not receive automated&#013;responses unless you are subscribed." >
                 <li class="">
                     <a href="mailto:geeqie@freelists.org"><i class="fa fa-twitter"></i> send message to mailing list</a>