Finish v3 API reference docs

[dstansby]

In #1918 I added structure for generating API docs, but only added the zarr namespace. We should add the other namespaces that need documenting too.

Does someone more involved in v3 have a list of which sub-modules should be added to the API docs?

[d-v-b]

If we are auto-generating content, then I would say we add everything to start, and then remove things later as needed.