Fix Makefile.am and make it easier to add a new language

	docs/C/gnomemm-website.xml: Add language 'es' and to the language list, and sort the list.
	docs/Makefile.am: Fix the problem of generate html pages for 'es' and 'zh_CN'. And made the code more generic, so if someone want to add a new language to Makefile.am, the only thing need to do is add the language to DOC_LINGUAS.