docs: give the deployment shapes one home and link to it

The three supported shapes were described in pieces across the swarm
docs, so a reader assembling them got a different answer depending on
which page they landed on. They live in services.md now, and the other
pages point at it.

Split out of #3719 at mara's request: that PR also rewrites the
snapshot-store doc, which is a separate argument.

docs/swarm/secrets.md

@@ -25,9 +25,13 @@ missing, and a secret minted on another container's first boot does not exist
 yet. Binding it would make one container wait on a file that waits on a
 container that starts after it.
 
-## The three topologies
+## Topologies, by who places secrets
 
-Every row below is read against one of these.
+Every row below is read against one of these. This is a different cut
+from the [deployment shapes](services.md#deployment-shapes) --- those
+say *where services run*, these say *who is responsible for a secret
+file being there* --- so the two lists do not line up one-to-one, and
+neither is a renaming of the other.
 
 | topology | what it means | who places secrets |
 |---|---|---|

docs/swarm/services.md

@@ -27,6 +27,36 @@ swarm-wide but **not optional**, being the canonical store for the meta
 flake and every agent's config repo, so it deploys with hyperhive itself
 and has no `enable` to derive from anything.
 
+## Deployment shapes
+
+Those two options are what makes the difference between deployments, so
+the shapes worth naming are the ones they produce:
+
+- **All-local.** Everything on one machine:
+  `enableAllLocalDefaults = true`. Setup is automatic apart from
+  choosing a domain and creating the first user.
+- **Services on the swarm controller host.**
+  `swarm.enableRequiredServices = true` there; the required services
+  deploy together on that host, with hives elsewhere.
+- **Fully spread out.** One container / VM / machine per service,
+  somewhere.
+
+**These are a set, not a ladder with a correct top, and in-between
+shapes are supported.** Each derived toggle can be set on its own (see
+above), which is what makes "all local except X" a configuration rather
+than an unsupported edge case. Nothing in hyperhive prescribes a
+deployment model, so a doc that treats one shape as the real one and
+the others as compromises is wrong about the system rather than
+opinionated.
+
+The practical consequence is that **co-location is normal**: in the
+first two shapes services share a host by design. Where a specific
+service has something to say about sharing a host --- extra
+requirements, or a cost worth weighing --- that belongs in the doc for
+that service, not here.
+
+## Services
+
 ### SSO (authelia)
 
 One authelia per swarm, in a `swarm-authelia` container, at