Add design schema for napari-spatialdata and interacting plugins

[LucaMarconato]

As suggested by @rtubelleza, it would be useful to have a document explaining the design and interactions of the various napari-spatialdata classes. This, paired with instructions on how third party developers can interact with the napari-spaitaldata plugin, would facilitate adoption.

In the process, we may need to expose new public APIs, an example is this one: #312.