Update Slack notification docs for workflow page install

[mintlify]

Summary

• Added a step to the dashboard Slack notification setup instructions noting that users can install the Slack app directly from the workflow configuration page
• Updated both English and Chinese workflow docs

Triggered by mintlify/mint@44392a4: Users can now connect Slack directly from the workflow creation or editing page without navigating to a separate settings page first.

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Note

Low Risk
Low risk documentation-only change updating step-by-step Slack notification setup guidance in English and Chinese.

Overview
Updates the workflows documentation to add an explicit first step for Slack notifications: if Slack isn’t connected, users can click Install slack app directly from the workflow create/edit page before enabling Notify on completion.

Applies the same instruction update to both agent/workflows.mdx and zh/agent/workflows.mdx.

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