docs: split swarm/deploy options into their own reference pages

services.hyperhive.swarm.* used to carry both swarm-wide facts (name,
domain, hives, ca) and the 'does THIS host run it' toggles for every
swarm service. deploy.nix already split those toggles out into their
own services.hyperhive.deploy.* namespace; this is the docs-side
follow-up now that swarm.* is genuinely swarm-wide-only.

nix/docs/default.nix: host.md drops the swarm/deploy subtrees
(builtins.removeAttrs on the already-picked host tree, not
lib.recursiveUpdate -- that merges rather than deletes, which would
have silently kept them on host.md); swarm.md and deploy.md are new
markdown pages, each its own pickSubtrees root. nix/packages/default.nix
exposes docs-swarm/docs-deploy as flake outputs alongside the existing
docs-host/docs-agent. docs/gotchas.md's Nix options reference section
and docs/swarm/README.md get pointers to the new split.

docs/swarm/README.md

@@ -5,6 +5,13 @@ coordinate across one or more hives. A single hyperhive instance
 running on one host is already a swarm (one hive). This doc covers
 the additional config needed when the swarm spans multiple hosts.
 
+For the full option reference rather than prose: `services.hyperhive.swarm.*`
+(swarm-wide facts, identical on every host) and `services.hyperhive.deploy.*`
+(this host's own deployment decisions — does *this* machine run grafana,
+the swarm controller, authelia, …) are separate generated pages, `nix
+build .#docs-swarm` / `.#docs-deploy` or the website's `/options/swarm.html`
+/ `/options/deploy.html`.
+
 ## Terminology
 
 - **hive** — a single hyperhive installation on one host. Has its