[docs] Put the file list back into the doxygen sidebar.

After cleaning up the sidebar, doxygen seemed to have skipped the step
of generating the webpages for the tutorials. Here, the menu is put back
into the sidebar to force doxygen to run the tutorials.

Author: hageboeck

documentation/doxygen/DoxygenLayout.xml

@@ -16,7 +16,7 @@
       <tab type="hierarchy" visible="yes" title="" intro=""/>
       <tab type="classmembers" visible="yes" title="" intro=""/>
     </tab>
-    <tab type="files" visible="no" title="">
+    <tab type="files" visible="yes" title="">
       <tab type="filelist" visible="yes" title="" intro=""/>
       <tab type="globals" visible="yes" title="" intro=""/>
     </tab>