Fix overloads in docs

[flying-sheep]

Need to move functions into API module because of sphinx-doc/sphinx#11410

Should be easy, e.g.

from ._sum import sum_  # the singledispatch function

@overload
def sum(…) -> …: ...
@overload
def sum(…) -> …: ...

def sum(a, axis) -> …:
   axis_check(axis)
   sum_(a, axis=axis)