Add Slack notifications documentation for workflows

[mintlify]

Summary

• Documents the new notify frontmatter field for workflows
• Adds section explaining how to configure Slack notifications for workflow completion/failure events
• Includes table of supported properties: channels, channel_ids, users, user_ids
• Updates both English and Chinese documentation

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Note

Low Risk
Docs-only changes that add configuration guidance; no runtime code paths or data handling logic are modified.

Overview
Documents a new notify frontmatter field for agent workflows and adds a Slack notifications section describing when notifications are sent, prerequisites for connecting Slack, and a YAML configuration example.

Includes a properties table for notify.slack (channels/users by name or ID) and mirrors the same documentation updates in zh/agent/workflows.mdx.

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