TYP: some more static definitions of methods for DatetimeIndex
xref #32100, https://github.com/pandas-dev/pandas/issues/31160#issuecomment-701244665
web and doc fails
```
2020-09-30T14:45:19.4780323Z /home/runner/work/pandas/pandas/pandas/core/indexes/datetimes.py:docstring of pandas.DatetimeIndex.rst:122: WARNING: autosummary: stub file not found 'pandas.DatetimeIndex.to_julian_date'. Check your autosummary_generate setting.
2020-09-30T14:45:19.4786910Z /home/runner/work/pandas/pandas/pandas/core/indexes/datetimes.py:docstring of pandas.DatetimeIndex.rst:122: WARNING: autosummary: stub file not found 'pandas.DatetimeIndex.isocalendar'. Check your autosummary_generate setting.
``` Thanks @WillAyd and @jbrockmendel for the review.

I'll do two commits, one to remove the decorator factory and another to remove the actual decorator. I think the static approach that I did originally is clearer, especially considering the handling of the different return types.

I raised this PR in connection with https://github.com/pandas-dev/pandas/issues/31160#issuecomment-701244665 to be more like https://github.com/pandas-dev/pandas/issues/31160#issuecomment-699193016

 > I think the static approach that I did originally is clearer,

and more error prone. Looks like made an error originally for `to_period`. but looks like can use `_simple_new_` for the different return types instead of the using the `Index` constructor, so maybe small perf improvements.

I think adding the doc decorator to each method could also be error prone, but at the same now allows substitution so could have separate html docs for Index and array methods.
 LGTM sure