ENH: Update Doxygen minimum required version to 1.8.4

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23452 3bd1e089-480b-0410-8dfb-8563597acbee
COMP: Fix doxygen warnings and tweak doxygen layout

This commit also updated the main page to include an "important link"
section. It has been inspired by the Blender main page.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23453 3bd1e089-480b-0410-8dfb-8563597acbee