Docs build: add napari + screenshots

[jni]

It would be nice if the docs included the napari viewer to demonstrate some of the reading and writing features, particularly now with scenes/transforms (#583).

[will-moore]

We added a napari screenshot into the docs at https://github.com/ome/ome-zarr-py/pull/582/changes (not released yet) but this didn't mention that you need napari-ome-zarr (or even napari).

[jni]

@will-moore sorry for the low context; I typed this up quickly during the ome-zarr-py meeting yesterday. Rather than manually making screenshots and committing them, this issue is about autogenerating the screenshots on docs build. I need to check the details but this can be done by copying the docs build configuration from https://github.com/napari/docs or, probably easier, https://github.com/jni/skan.