docs: put the credentials prose on the overview and the tables on the reference

The folded-in secrets.md text sat after the reference's table, so the
reference was half prose. The overview now holds all the prose (where a
secret lives, topologies, the swarm- and hive-level notes, how a reader
reaches the store, the browser UI, adding a secret) and the reference holds
only the tables, each pointing at the section that explains it. No text
dropped; links that asked for the explanation now go to the overview, and
the ones that asked for a list or a row go to the reference.

nix/host-modules/swarm-nats.nix

@@ -360,7 +360,7 @@ in
   # the host running it decides is below — which responder build it runs and
   # the store identities its units present. `enable` already lives in
   # ./deploy.nix, which also carries the renames. The callout's keys are no
-  # option at all: they live in the secret store (docs/swarm/credentials-reference.md).
+  # option at all: they live in the secret store (docs/swarm/credentials.md).
   options.services.hyperhive.deploy.nats = {
     authPackage = lib.mkOption {
       type = lib.types.package;