Write explanatory documentation on ~~properties~~ features

[andy-sweet]

📚 Documentation

We have some existing content (both on napari.org and in the example scripts) that references and uses properties to do things like color/style shapes and add text annotations. However, we're lacking some high level documentation that explains how users and developers should think about properties - i.e. what information they capture and what the general mode of interaction is.

We should only start writing this documentation once there is more progress on napari/napari#2866 and we have a better idea of exactly what the unified behavior and interaction of properties will look like across all layers.