Improve the appearance of Texinfo's HTML output with an external stylesheet.

2013-08-26  Didier Verna  <didier@didierverna.net>

	* doc/janix-texinfo.css: New file.
	* doc/Makefile ($(HTML_DIRS) $(REF_HTML_DIRS) $(WEBREF_HTML_DIRS)):
	Use Janix's Texinfo CSS file for HTML output.