Fix import statements in statcast docs

[wfordh]

Fixing issues with documentation for statcast docs brought up by #225

[bdilday]

I think it'd be a good idea to expose these at the top level, example, in pybaseball/__init__.py

from .statcast_batter import (
    statcast_batter,
    statcast_batter_exitvelo_barrels,
    statcast_batter_expected_stats,
    statcast_batter_percentile_ranks,
    statcast_batter_pitch_arsenal,
)

[wfordh]

Ah so that's why I could import statcast_batter() directly and couldn't with these. I can add the functions into pybaseball/__init__.py like that for all of them and then revert back to the original documentation

[schorrm]

LGTM, @bdilday all good?