feat: about extendr

we need more copy on "what" extendr is. Right now from here its somewhat clear what extendr is, but not super.

<img width="1385" height="742" alt="Image" src="https://github.com/user-attachments/assets/967e879f-868c-4083-8f03-0ba8b23dc878" />

Either an `About` or a section further down on this page that describes in a more "marketing" way what extendr is. 
