man: add stub sd-bus(3)

man: add stub sd-bus(3)

We have 126 broken links to sd-bus.html, it's nice to fix that.
Current version is mostly a stub, with a long list of links to other
pages. I think that's fine, especially that sd-bus might evolve
quite a bit before it is made public.

Not all of linked pages are written. Still missing:
sd_bus_can_send
sd_bus_get_name_creds
sd_bus_get_owner_creds
sd_bus_message_can_send
sd_bus_message_get_creds
sd_bus_message_set_allow_interactive_authorization
sd_bus_send
sd_bus_set_address
sd_bus_set_description
sd_bus_start
sd_event_set_prepare
sd-device
systemd.busname