Update napari.yaml, include napari-hub category labels

[GenevieveBuckley]

Adds "Segmentation" and "Annotation" category labels for our future napari-hub plugin page.

See https://napari.org/stable/plugins/manifest.html for valid categories.
Currently allowed categories are: Acquisition, Annotation, Dataset, Image Processing, IO, Measurement, Segmentation, Simulation, Themes, Transformations, Utilities, Visualization

Related to (but does not close issue) #530

[GenevieveBuckley]

FYI: it seems that the "Image Modality" labels can only be controlled by adding a .napari-hub/config.yml file. This is because this key is not (yet?) supported by the napari manifest napari.yaml file.

We could add that too, I guess, but it does feel messy having so many config files scattered around.

I only found one plugin on the hub using the Image Modality labels (probably because it is not well-documented how to set them up). It was the napari-biomag-annotator plugin, see the example .napari-hub/config.yml file here.

Note: That .napari-hub/config.yml file does list a lot more labels than I see actually being used by the napari-hub page, so I'm not sure if some of these are effectively not being used. It's possible only specific values are accepted, but I can't find good documentation about it, so this one example is the best we have to go on.

[constantinpape]

Thanks @GenevieveBuckley.

Regarding the Imaging Modalities: I would not add this for now; we can still add it later on.