docs: automatic key failover configuration

[ssncferreira]

Description

• How to configure multiple keys (comma-separated env vars, examples)
• Startup validation rules (max 5, no duplicates, no empty keys)
• Failover behavior overview (primary-with-fallback, 429/401/403 triggers)
• Key health visibility (log entries and metrics admins can monitor)
• Limitations (no hot-reload, Bedrock/Copilot excluded)