You are a SaaS and email marketing automation expert. Write a detailed, 2500-word guide on onboarding triggers. The guide must cover: 1) Signup triggers, 2) First login triggers, 3) Feature adoption triggers, 4) Inactivity triggers during onboarding, 5) Milestone triggers, 6) Setting up trigger-based onboarding, 7) Personalization based on behavior, 8) Examples of onboarding triggers, 9) Measuring activation, 10) A/B testing triggers.