web: Update Doxyfile.

This file was created long ago with doxygen version 1.5.4. The system
on which the public web pages are generated runs Ubuntu 14.04 which
ships doxygen-1.8.6. This doxygen version complains as follows:

	Warning: Tag `DETAILS_AT_TOP' at line 162 of file web/Doxyfile has become obsolete.
	To avoid this warning please remove this line from your configuration file or upgrade it using "doxygen -u"
	Warning: Tag `SHOW_DIRECTORIES' at line 424 of file web/Doxyfile has become obsolete.
	To avoid this warning please remove this line from your configuration file or upgrade it using "doxygen -u"
	Warning: Tag `HTML_ALIGN_MEMBERS' at line 727 of file web/Doxyfile has become obsolete.
	To avoid this warning please remove this line from your configuration file or upgrade it using "doxygen -u"

This patch is the result of running doxygen -u web/Doxyfile. With the
updated file the above warnings go away, and the pages look identical.