Added Slack agent onboarding and acknowledgment documentation

[cdxker]

Documented the first-time setup DM flow and private message acknowledgment behavior for the Slack agent. Added placeholders for screenshots to be uploaded later.

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Created by Mintlify agent

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Note

Updates agent/slack.mdx with onboarding and usage details for the Slack agent.

• New: First-time setup section describing the initial DM from the Mintlify app after connecting
• New: Message acknowledgment section explaining private acknowledgments in thread when mentioning @mintlify
• Adds <Frame> placeholders for screenshots in both new sections

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