docs: add alert rule configuration example to alerts manual

[databufflabs]

Description

The alerts manual (docs/使用手册/告警.md) describes concepts and menu navigation but lacks concrete configuration examples. New users often ask "how do I set up a simple error rate alert?" — a working YAML/JSON example would help them get started immediately.

Task

Add a new section "配置示例" (Configuration Example) to the alerts manual with a practical alert rule example, such as a threshold alert for service error rate > 5% sustained for 5 minutes.

Include:

• The rule configuration fields (metric, threshold, duration, etc.)
• A screenshot of the alert creation UI (optional, bonus)
• Notes on how the alert appears in the Events list

Files to change

• docs/使用手册/告警.md (and optionally docs/使用手册/告警_en.md for the English version)

Acceptance criteria

• New "配置示例" section added to docs/使用手册/告警.md
• Example includes a realistic threshold rule (e.g., error rate > 5% for 5 min)
• Fields are explained in plain language
• English version docs/使用手册/告警_en.md updated with equivalent content (optional but preferred)

Skill requirements

• Markdown
• Basic understanding of alerting/threshold concepts

Resources

• docs/使用手册/告警.md — the manual to enhance
• docs/架构设计/告警.md — architecture doc with evaluation details