docs: add alert rule configuration example

[Titanism0v0]

Summary

• Add a concrete service.error.pct > 5% threshold alert example to the Chinese alerting manual.
• Document the rule fields, JSON configuration, five-minute lookback semantics, and event/alert behavior.
• Add the equivalent English documentation.

Validation

• git diff --check
• Parsed all JSON code blocks in both alerting manuals successfully.
• Compared the head against databufflabs/databuff:master; only the two alerting manual files are changed.

Closes #11