kernel-doc: alphabetically-sorted entries in index.html of 'htmldocs'

Make docbook index.html contain sorted output.

I prefer to let the computer do it.  This also avoids
people not reading the comment(s).

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>