DOC: DatetimeArray.std
`DatetimeIndex.std` is missing a doc-string. Add doc-string to `DatetimeArray.std` as `DatetimeIndex` is re-using `DatetimeArray.std`.


There are a few more keywords, but they seem to be not implemented (numpy-compatiblity?)

```py
pd.DatetimeIndex([1, 2]).std(out=[])
ValueError: the 'out' parameter is not supported in the pandas implementation of std()

pd.DatetimeIndex([1, 2]).std(keepdims=True)
ValueError: the 'keepdims' parameter is not supported in the pandas implementation of std()

pd.DatetimeIndex([1, 2]).std(dtype=np.dtype("datetime64[ns]"))
ValueError: the 'dtype' parameter is not supported in the pandas implementation of std()
```

For some reason, this doc-string is needed for #43828.
Getting

> /home/runner/work/pandas/pandas/pandas/core/indexes/datetimes.py:docstring of pandas.core.indexes.datetimes.DatetimeIndex:124: WARNING: autosummary: stub file not found 'pandas.DatetimeIndex.std'. Check your autosummary_generate setting.


even though `pd.core.indexes.datetimes.DatetimeIndex([1]).std.__doc__` returns the doc-string.


edit: Running `python doc/make.py --single "pandas.DatetimeIndex.std" --num-jobs 1` also picks up on the doc-string. I think it wants an 'std' entry in doc/source/reference/indexing.rst DatetimeIndex.std > I think it wants an 'std' entry in doc/source/reference/indexing.rst DatetimeIndex.std

Thank you @jbrockmendel , now it seems to be happy :)