doc: replace publican with xmlto

Publican isn't packaged for some distros, xmlto is a lot more common. Most of
what publican provides for us is the stylesheet anyway, so we can just use
xmlto and the publican stylesheet to get roughly the same look.

PDF and XML generation has been dropped, this needs a bit more more effort
than a mere switchover to xmlto.

The top-level directory structure imposed by publican is kept for now
(specifically the Wayland/en-US/html tree). This makes it easier to transition over
for packagers. Note that the list of files inside has changed.

CSS files are taken from publican to keep a uniform look compared to previous
documentations. Stylesheets are licensed under CC0 1.0 Universal license, see
publican/LICENSE:

1.  Files in the datadir/Common_Content directory and its subdirectories are
    licensed under the CC0 1.0 Universal license.

    To the extent possible under law, the developers of Publican waive all
    copyright and related or neighboring rights to the files contained
    in the datadir/Common_Content directory and its subdirectories.

Acked-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>