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.

swarm-secret-client/README.md

@@ -7,7 +7,7 @@ field its bytes are in, and how this deployment's environment becomes a
 logged-in client.
 
 The surrounding picture — which secret is minted where, and why delivery is a
-copy rather than a bind mount — is [`docs/swarm/credentials-reference.md`](../docs/swarm/credentials-reference.md).
+copy rather than a bind mount — is [`docs/swarm/credentials.md`](../docs/swarm/credentials.md).
 
 ## Why a crate and not a module per binary