Annotate API-exposed Items from pandas.core.api
We currently expose all of the following items from pandas.core.api via the API:

```python
from pandas.core.api import (
    # dtype
    Int8Dtype, Int16Dtype, Int32Dtype, Int64Dtype, UInt8Dtype,
    UInt16Dtype, UInt32Dtype, UInt64Dtype, CategoricalDtype,
    PeriodDtype, IntervalDtype, DatetimeTZDtype,

    # missing
    isna, isnull, notna, notnull,

    # indexes
    Index, CategoricalIndex, Int64Index, UInt64Index, RangeIndex,
    Float64Index, MultiIndex, IntervalIndex, TimedeltaIndex,
    DatetimeIndex, PeriodIndex, IndexSlice,

    # tseries
    NaT, Period, period_range, Timedelta, timedelta_range,
    Timestamp, date_range, bdate_range, Interval, interval_range,
    DateOffset,

    # conversion
    to_numeric, to_datetime, to_timedelta,

    # misc
    np, Grouper, factorize, unique, value_counts, NamedAgg,
    array, Categorical, set_eng_float_format, Series, DataFrame,
    Panel)
```

A pseudo-prioritized list of annotations I think we need out of this would be the below. Open to suggestions on how to prioritize and obviously community PRs are very welcome!

 - [ ] DataFrame
 - [ ] Series
 - [ ] Index
 - [ ] MultiIndex
 - [ ] Categorical
 - [ ] CategoricalIndex
 - [ ] Datetimelike indices
 - [ ] Numeric indices

 ...

These don't necessarily need to be completed in order. Will continue to expand checklist as we tackle more items so if you see something you'd like to tackle feel free to call it out