REF: make modellib private + support numpy docstring style in sphinx docs

[kaczmarj]

make modellib private, and expose the api at the top level. also add napoleon and intersphinx extensions to sphinx docs.

from ._modellib.models import get_model_weights  # noqa
from ._modellib.models import list_all_models_and_weights  # noqa
from ._modellib.models import register_model_weights  # noqa
from ._modellib.run_inference import run_inference  # noqa
from ._modellib.run_inference import WholeSlideImagePatches  # noqa
from ._modellib.transforms import PatchClassification  # noqa