docs(guide): mark USERGUIDE.md deprecated

It is too far out of date to be worth repairing. It describes identity keys
derived from a username and password, one `shared_dir` per group with an
`uploads/` quarantine, ChaCha20 as the content cipher, a hub that stores users'
public keys and the wrapped group keys, and a member wrapping that key for
another member — which is finding H3, in the section that explains why the hub
cannot read your files.

The banner lists each of those against what is actually true, so that no section
below it is mistaken for current, and points at MESHBAY_DESIGN.md and
MESHBAY_NODE_PROTOCOL.md instead. It also records what the document predates
entirely: encrypted chat, the sealed index and upload path, transfer leases,
device linking, the application framework.

Repairing it section by section is refused deliberately. Enough of it is wrong
that a reader cannot tell the sound parts from the stale ones, which is worse
than having no guide, and fixing one section leaves exactly that problem in
place. The previous commit — which translated two French passages and corrected
the errors immediately around them — is dropped for the same reason: it made a
small part of a misleading document accurate, which makes the whole harder to
distrust, not easier.

There is no replacement user guide today. That gap is real and is better stated
than papered over.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YVoHVCcfBqud6ZjG4db3y7

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