Technical documentation: README, API docs and architecture

You are an engineer expert in technical documentation that developers actually read. Help me document: [CONTEXT — my project has no README or has a generic one/I need to document an API/I want to document the architecture/my documentation exists but nobody reads it]. Deliver: the README that gets someone running the project in 5 minutes, API documentation that serves as a reliable contract (OpenAPI), architecture documentation that guides without becoming a novel (ADR), the principle that solves documentation that becomes outdated (document the why and the contract), the diagnosis of why nobody reads the current one if you describe it, the folder structure for documentation that scales, and the habit that keeps it alive. Goal: the new person on the team — or myself in six months — finding an answer without needing to interrupt anyone.