docs(workflows): add email engagement events reference

[dmarchuk]

Summary

Adds a new docs page documenting the seven $messaging_email_* engagement events emitted when a project opts in to the new Workflows → Engagement events setting (added in posthog#50122, enabled by the consumer in posthog#60212).

Linked from the new Settings entry in the PostHog app so users have a single canonical reference for what each event means and how to build insights from them.

What's in the page

• One-paragraph intro covering the opt-in nature of the setting
• How to enable (Settings or inline from the email step)
• Table of all seven events (_sent, _delivered, _opened, _link_clicked, _bounced, _blocked, _failed) with the trigger condition for each
• Common analysis patterns (trends, funnels, retention, dashboards)
• Properties carried on each event (workflow_id, workflow_action_id, invocation_id, message_id)
• Relationship to existing workflow metrics
• Privacy/cost note

Test plan

• Render locally and confirm the table + sidebar entry look right
• Sidebar entry shows up under Workflows → Resources

🤖 Generated with Claude Code

[github-actions]

Deploy preview

Status	Details	Updated (UTC)
🟢 Ready	View preview	May 29, 2026 12:31PM

[github-actions]

Vale prose linter → found 4 errors, 0 warnings, 0 suggestions in your markdown

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contents/docs/workflows/engagement-events.mdx — 4 errors, 0 warnings, 0 suggestions

Line	Severity	Message	Rule
46:17	error	Hi, Andy here... use an en dash ( – ) with spaces. On Mac, holding down the Option and hyphen key will give you an en dash.	PostHogBase.EnDash
47:24	error	Hi, Andy here... use an en dash ( – ) with spaces. On Mac, holding down the Option and hyphen key will give you an en dash.	PostHogBase.EnDash
48:19	error	Hi, Andy here... use an en dash ( – ) with spaces. On Mac, holding down the Option and hyphen key will give you an en dash.	PostHogBase.EnDash
49:16	error	Hi, Andy here... use an en dash ( – ) with spaces. On Mac, holding down the Option and hyphen key will give you an en dash.	PostHogBase.EnDash