ENH: classmethods for pd.period_range & pd.date_range
Floating an idea: `pd.PeriodIndex.from_range` & `pd.DatetimeIndex.from_range` as `classmethod`s to mirror `pd.period_range` & `pd.date_range` respectively. This encapsulates constructors on the class.

Also could have `pd.DatetimeIndex.from_dataframe` for https://github.com/pydata/pandas/pull/12967
