docs+nix: fix stale certFingerprint/HYPERHIVE_PEERS references (hyperhive#3294)

Follow-up to the plumbing removal: docs/swarm/README.md gets the
biggest rewrite (drops the whole "Fingerprint format" section, fixes
the runtime-effects list, the WireGuard config example + "what the
mesh does" bullet), docs/gateway.md and hive-gateway/options.nix drop
4 "needs no certFingerprint" mentions, swarm-peers-removed.nix's
migration-warning text no longer tells an upgrading operator to carry
a field over that no longer exists, swarm.nix/swarm-wireguard.nix/
swarm-controller.nix/swarm-controller's main.rs get comment fixes
where they described the now-removed HYPERHIVE_PEERS shape. Also
caught one more stale "peer hives" mention in docs/web-ui/README.md's
SW4RM tab description that the first pass on this issue missed.

nix/host-modules/swarm-controller.nix

@@ -301,10 +301,9 @@ in
       # as one attrset and never assigned individually.
       environment = {
         SWARM_CONTROLLER_SOCKET = cfg.socketPath;
-        # The swarm's hive directory, JSON-encoded — same shape hive-c0re
-        # already builds for HYPERHIVE_PEERS (../hive-c0re/environment.nix),
-        # just the full directory (this daemon has no "self" hive to
-        # exclude, unlike a per-hive c0re's peer list) rather than
+        # The swarm's hive directory, JSON-encoded — the full directory
+        # (this daemon has no "self" hive to exclude, unlike
+        # `swarm.peerHives`, `swarm.hives` minus this hive) rather than
         # peers-minus-self. Consumed by `GET /api/hives`
         # (swarm-controller/src/main.rs::load_hives).
         SWARM_CONTROLLER_HIVES = builtins.toJSON (