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.
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