New docs/swarm/ui.md (split-page shape, per the docs rule), linked from
the swarm README and added to the gateway's vhost map.
Leads with the step that separates 'protected' from 'locked out':
swarmctl user add <you> --group operators. auth_request asks whether
there is a session; the access_control rule is what makes it mean
operator, and an account created before the rule existed has no groups.
Also records the four wiring sites a swarm service name needs, with the
certificate one called out - serviceDomains is both the sub-CA's
nameConstraints set and the leaf's SANs, and the apex is a sibling of
the other three rather than a parent, so nothing issues for it
implicitly.
