Configure github pages

Configure github pages

Upload the doxygen output as artifact from the linux build and use that from
the pages job where we combine the static website with our newly build HTML
docs.  The GitHub actions/download-artefact doesn't work across workflows so
we use the other popular one that can do this. The rest of the job is
basically copy/paste from the "Static HTML" example GitHub provides.

To make this useful as drop-in replacement, replace the one fixed link to the
API docs a relative one.

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