Events docs

[jvstme]

Describe events in dstack docs — possibly on separate Concepts page.

Consider including:

• Description of what events and event targets are.
• Viewing events in the UI.
• Viewing events in the CLI.
• Configuring events TTL.

Also resolve this TODO in the CLI reference.

[github-actions]

This issue is stale because it has been open for 30 days with no activity.