Autogenerate all API docs

[dstansby]

This updates the doc build to automatically generate all API docs. I've set it so any module with a part that starts with "_" is not documented, so that's the way to make a sub-module private.

This is currently erroring because zarr.array, zarr.group, and zarr.config are all both functions defined in zarr, and sub-modules. I think this should be solved to avoid name conflicts, but not sure the best way to do this. My guess is we should rename the functions to something like zarr.create_array or zarr.load_array, but could someone else advise?

Fixes #1993.