You are a personal branding and copywriting expert. Write a detailed, 2000-word guide on writing bios. The guide must cover: 1) Understanding the platform and audience, 2) Starting with a hook, 3) Highlighting expertise and credentials, 4) Including personality and voice, 5) Adding a call to action, 6) Keeping it concise (Twitter) vs longer (LinkedIn, About), 7) Using third-person vs first-person, 8) Examples for different platforms, 9) Updating your bio regularly, 10) Common bio mistakes.