Add more documentation about `CanvasProvider` as it's required for proper styling

[RayRedGoose]

🚀 Feature Proposal

We don't have a good explanations about CanvasProvider and how works. But it's required after adding cache handling to it for proper styling. CanvasProvider can be easily missed and its concept can't be fully understood.

Motivation

It will help our consumers to understand better why CanvasProvider need and how to use it.

Current CanvasProvider info

Welcome page Emotion cache discussion Theming Readme

[alanbsmith]

Seems like we still need to do this.