Update README.md

[glenn-jocher]

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Comprehensive guide added for Ultralytics documentation setup, local serving, and deployment.

📊 Key Changes

• Detailed instructions for installing the Ultralytics package in developer mode.
• Steps for building and serving the documentation site locally with MkDocs.
• Notes on how to deploy the documentation site using various platforms like GitHub Pages and options for setting a custom domain.
• A representative screenshot for setting a custom domain on GitHub Pages included.

🎯 Purpose & Impact

• User-Friendly Guidance: Helps both new and experienced users install and contribute to the Ultralytics documentation with ease.
• Immediate Reflection: Enables contributors to instantly see the effect of their changes locally, promoting efficient testing and debugging.
• Streamlined Deployment: Clarifies the deployment process, making it easier for maintainers to update and manage the live documentation site.
• Visual Aid: The added screenshot serves as a quick reference for configuring a custom domain, enhancing user understanding.