manual: Add small overview on the Hetzner backend.

As there aren't quite a lot of options to set for the Hetzner backend,
this small introduction should at least ensure people know the backend
exists and give a short overview on how to get started.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
manpage: Document HETZNER_ROBOT_* env variables.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>