Switching from doxygen to sphinx broke a bunch of links because doxygen used
whatever the argument to @page was - and that usually had underscores. Sphinx
uses filenames (which use dashes) so now we have a bunch of old links going to
a 404. For the transition period at least, insert a custom 404 page for each
of those to tell users this doesn't exist anymore.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>