docs: external-party outbound transfer (registry config + runbook + API)

[sadiq1971]

Background

The outbound-to-external-party path has operational nuances (2-phase offer/accept, expiry, registry dependency, claim-back) that need to be written down for integrators and operators.

Scope

• API docs for the new endpoints (/api/v2/transfer/external/prepare|execute, custodial /api/v1/transfer/external), including the 2-phase semantics: success = offer submitted, not settled.
• Config docs: token.external_tokens (InstrumentAdmin → RegistryURL) must include the recipient token's registrar for outbound; note that registry reachability is a hard dependency of prepare (registry down → prepare fails).
• Operator runbook: what happens when the recipient never accepts (offer expires, holdings unlock) and how the sender reclaims via the claim-back flow (feat: claim back expired offers #292).
• Follow the existing docs/*-design.md pattern.

Acceptance

• docs/ page covering the outbound external-party transfer path end-to-end.
• Config reference updated with the external-token registry requirement for outbound.

[sadiq1971]

Tracked by epic #321 (USDCx transfer to external parties via party id).