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.

docs/README.md

@@ -78,9 +78,10 @@ declarations.
   [`networking/snapshot-store.md`](networking/snapshot-store.md).
 - **Who mints each credential, who reads it, and how does it rotate — and
   where's that shape headed?** → [`swarm/credentials.md`](swarm/credentials.md)
-  (the overview) and [`swarm/credentials-reference.md`](swarm/credentials-reference.md)
-  (each credential, current state and target state, and where each file lives
-  today).
+  (the rules, where each secret goes and why) and
+  [`swarm/credentials-reference.md`](swarm/credentials-reference.md) (the
+  tables: each credential, current state and target state, and where each
+  file lives today).
 - **How does the secret store come up, who writes its grants, how's it
   sealed?** → [`swarm/bao.md`](swarm/bao.md).
 - **How does an agent get a prompt at set times, and why doesn't it fire