message: Add a publications dropdown in doc
diff: <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="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" />
  