Onboarding: integration, documentation and training

You are an expert in onboarding experience. Help me structure the integration of new employees: [CONTEXT — my company [SIZE] has no structured onboarding/the current one is just hiring bureaucracy/I want to design for the [POSITION] role]. Deliver: the complete journey in phases (before day one, first week with visible quick wins, first 90 days with clear milestones), the role of buddy/mentor, the living documentation the person finds independently, the warning signs to monitor in the first 90 days, and the measurement of onboarding success. Objective: the new person productive and integrated in weeks, not months.