Every docs directory moves under 1-getting-started, 2-handbook,
4-implementation or 5-project; the options reference (section 3) comes from
the website and has no directory here. trust-boundary stays whole under the
handbook for now. web-ui, swarm and tools split between handbook and
implementation, and the command-line references move to
2-handbook/reference. Links, path references in code and nix, and the docs
index follow the moves; no page content changes.
The dashboard vhost, the hive domain vhost and the dashboard's local
name move to hive-c0re/vhosts.nix; the matrix .well-known discovery and
the legacy /matrix/ redirect move to glue-matrix-hive-discovery.nix,
which adds them to the hive domain vhost. Both declare through
gateway.vhosts. hive-gateway/vhosts.nix keeps the catch-all only, and
the gateway no longer reads deploy.matrix or c0re.servedFrontend, nor
deploy.hive-controller.enable for its vhosts.
Part of #3742.
