scripts/kernel-doc: added support for html5

New output option html5 writes validating HTML5 and adds
CSS classes ready to be selected by third-party stylesheets.
HTML ids have been added to block-level elements "article" for
direct reference of particular objects via URL.
Signed-off-by: Dan Luedtke <mail@danrl.de>
Acked-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Michal Marek <mmarek@suse.cz>