More reorganization of the documentation.

o Updated the Doxyfile template to the latest version of doxygen (1.8.6).  This shouldn't cause a problem; older versions will throw warnings about unrecognized options, but they should still build the documentation.
o Added the 'netcdf' icon to doxygen-generated documentation.
Merge branch 'master' of https://github.com/Unidata/netcdf-c