docs: Update html docs for libwayland

I'm not sure what the new procedure for this is, but since the documentation posted under https://wayland.freedesktop.org/docs/html/ is out of date, I'm making this pull request to renew it.

The most recent update of this folder was pre- Gitlab porting (June 2018). Between version 1.18.0 and git master, there have not been any hard (interface-version-increasing) protocol changes for wayland.xml, although there have been clarifications made to wayland.xml and the doc/publican/sources documentation.