swarm-ui: build the forge link from swarm.forge.domain

The swarm-controller builds the Forge quick link from
services.hyperhive.swarm.forge.domain, replacing hive-forge/default.nix's
per-host entry, so all seven swarm-service links come from swarm-level
options.

Also drops the remaining references to the removed matrix GUI switch:
the HiveUrls / Urls / hive_urls docs, the hivectl.md `open` note and
the gateway.md vhost-map rows, which name `gatewayHost` instead. The
grafana, victoriametrics and victorialogs modules' comments no longer
mention a quick-link they do not define.

Refs #4885

nix/module-eval/swarm-services-switch.nix

@@ -117,7 +117,7 @@ let
       Logs = s.victorialogs.domain;
       Matrix = s.matrix.gatewayHost;
       Bao = s.bao.ui.domain;
-      # forge: added once hive-forge/default.nix drops its per-host link
+      Forge = s.forge.domain;
     };
   swarmServiceLinksOf =
     cfg: