Trailing #615 + #620 rebase fix: `nix/docs.nix`'s stub NixOS eval still
referenced the old `hyperhive.{forge,matrix}.enable` paths that #615
moved under `services.hyperhive.*`. Update to match + also force-disable
the new `services.hyperhive.gateway.enable` so the docs eval doesn't
spawn the gateway container as part of `nix build .#docs`.
`packages.docs{,-host,-agent}` and `checks.docs` all evaluate cleanly
on the post-#615 / post-#620 shape verified via `nix flake check`.
34f0c11936