docs: split swarm.md into a directory, starting with the services page

`docs/swarm.md` becomes `docs/swarm/README.md` and the shared-services
material moves to `docs/swarm/services.md`, following the shape
`docs/turn-loop/` and `docs/web-ui/` already use. The README keeps a
pointer so the reading path is unbroken.

Every referrer moved with it — five docs pages, two option descriptions
in swarm.nix, and CLAUDE.md's reading path. A pointer to a file that
moved is worse than one to a file that was deleted: the content still
exists, so the reader concludes the note is wrong rather than the path.

nix/host-modules/swarm.nix

@@ -33,7 +33,7 @@
               Generate with `openssl x509 -noout -fingerprint -sha256`,
               then strip the colons and prepend `sha256:`. A malformed
               value is ignored with a warning rather than weakening
-              trust. See docs/swarm.md for the full recipe.
+              trust. See docs/swarm/README.md for the full recipe.
 
               Scopes only to hive-c0re's own peer HTTPS checks — it does
               NOT help Matrix federation (tuwunel validates against its
@@ -62,7 +62,7 @@
               inside the hive (agents + the Matrix container), never the
               host system trust store. Mutually complementary with
               `certFingerprint`; set `caCert` for the federation case. See
-              docs/swarm.md.
+              docs/swarm/README.md.
             '';
           };