ENH: warn Series.interpolate(method='index') with unsorted index
`Series.interpolate(method='index')` use `numpy.interp` under the hood, which, from numpy doc, _Does not check that the x-coordinate sequence xp is increasing. If xp is not increasing, the results are nonsense._ [(link)](https://docs.scipy.org/doc/numpy/reference/generated/numpy.interp.html)
Here adds a warning to warn the user about this.

- [x] closes #21037
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Can this be fixed instead of producing a warning? Yes it can, in the cost of introducing dependency on scipy. (like other interpolate methods) We can't use `pd.where(..., df.index)` to fill from the index? You mean we sort it, interpolate, then fill it back?  this may leads to undefined behavior for duplicated indexes, better leaves the responsibility to scipy IMHO. :joy:  > You mean we sort it, interpolate, then fill it back? this may leads to undefined behavior for duplicated indexes, better leaves the responsibility to scipy IMHO. 😂

then would just raise rather than a warning.  So the preferred approach would be stick to `numpy.interp` but we do the sort for it right? No problem. I will close this one and do it in a new branch. Thanks for your suggestions. i think either we sort then reorder to get the original 
or raise