Documentation for Friction core architecture

I wrote a document to help me understand how the general class hierarchy works in Friction: https://www.figma.com/design/s9YWnyj8CNh2IEstjJa3fZ/Friction2d-architecture-roadmap?node-id=0-1&t=h4P0uJ3jRXu4yK5D-1

And proposing some changes.