docs(middleware): document response pre-return workflow

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Summary

Document the HTTP response pre-return contract and reorganize the supervisor middleware guide around request, response, and WebSocket runtime flows. Keep the gateway configuration page focused on the TOML surface and delegate detailed middleware behavior to the middleware guide.

Stacked on #3074, which implements the response runtime and relay integration. The lower stack layers are #3073 for the interface and #3072 for shared header mutation authority.

Related Issue

Closes #2691

Changes

• document response preflight, headers-only, whole-body, streaming, trailers, framing repair, and failure semantics
• group the middleware guide into runtime flow, configuration, and operations sections
• document request, response, and trailer header authority with the shared validator model
• point the guide at the existing content-guard example
• reduce the gateway configuration section to the TOML fields and delegate contract details to the middleware guide
• reconcile policy, architecture, RFC, authentication, and protocol-extension references

Testing

• mise run pre-commit

Checklist

• Published documentation describes the user-visible behavior
• Gateway configuration retains the complete field surface without duplicating the middleware guide
• RFC and architecture references match the implemented contract
• Commit is signed off for DCO

Stack

1. feat(middleware): broaden HTTP header mutation authority #3072 — shared header mutation authority
2. feat(middleware): define HTTP response pre-return interface #3073 — response protobuf and transport interface
3. feat(network): inspect HTTP responses before return #3074 — response runtime, relay integration, and existing example
4. docs(middleware): document response pre-return workflow #3075 — documentation and middleware guide reorganization

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