Merge: Add a publications dropdown in doxygen documentation page

This PR adds a dropdown for Ginkgo's publications on the documentation page, which can showcase the white paper of Ginkgo rendered nicely as a webpage, courtesy of arxiv-vanity.com

Related PR: #724

doc/DoxygenLayout.xml

@@ -5,7 +5,11 @@
     <tab type="mainpage" visible="yes" title=""/>
     <tab type="user" visible="yes" title="Tutorial" url="https://github.com/ginkgo-project/ginkgo/wiki/Tutorial:-Building-a-Poisson-Solver" />
     <tab type="user" visible="yes" title="Examples" url="@ref Examples" />
-    <tab type="user" visible="yes" title="Citing Ginkgo" url="@ref citing_ginkgo" />
+    <tab type="user" visible="yes" title="Publications" url="@ref citing_ginkgo" >
+      <tab type="user" visible="yes" title="Citing Ginkgo" url="@ref citing_ginkgo" />
+      <tab type="user" visible="yes" title="Ginkgo White Paper" url="https://www.arxiv-vanity.com/papers/2006.16852/" />
+      <tab type="user" visible="yes" title="Slides and Outreach" url="https://github.com/ginkgo-project/outreach" />
+    </tab>
     <tab type="user" visible="yes" title="Contributing To Ginkgo" url="@ref contributing_guidelines" />
     <tab type="user" visible="yes" title="Using Ginkgo" url="@ref install_ginkgo">
       <tab type="user" visible="yes" title="Installing Ginkgo" url="@ref install_ginkgo" />