docs: clarify generated metrics task toggle in default config and docs

[Copilot]

The configuration docs and defaults did not clearly expose the flag controlling auto-generated metrics tasks, making it easy to miss when aligning config behavior with recent changes.

• Config default: Added explicit configs.allow_generated_metrics_tasks: false to agent.yml with a clarifying comment.
• Docs (EN/zh): Documented the new toggle in both configuration index pages with a brief description of when to enable it (e.g., Kubernetes auto-generated metrics).

Example snippet:

configs:
  managed: true
  panic_on_config_error: false
  allow_generated_metrics_tasks: false # allow auto-generated metrics tasks (e.g. k8s)

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