Add SEO page description

This project is the [top Google result for "rt simulator"](https://www.google.com/search?q=rt%20simulator), but that page description could use some work:

<img width="487" alt="Image" src="https://github.com/user-attachments/assets/82f7ef26-25bc-428f-bcb1-d8cfe5bbdaa8" />