docs(request-trace-uploader): document deployment and operations

[kristinapathak]

Why

The request-trace-uploader needs a clear deployment and operational contract before users replace the existing sidecar.

Scope

• Document required Dynamo request-tracing settings for request_payload records and the trace and audit file contract.
• Document the sidecar image, shared spool, read-only credential mount, state and quarantine directories, health probes, and configuration settings.
• Document the uploader own OTLP logs, traces, and metrics through BYOO signal-specific endpoints. Do not describe a Prometheus scrape path.
• Document S3 source lifecycle, retry and failure behavior, NCA audit payload drop behavior, redaction, and safe credential rotation.
• Add operator guidance for backlog, oldest pending age, retries, terminal failures, quarantine, stale success, configuration, and credential alerts.
• Add a concise migration guide from the existing shell sidecar once the uploader is ready.

Tests

• Validate all configuration names, defaults, and behavior examples against the implementation.
• Render deployment examples and verify no secret values or request payloads appear.

Parent: #1004