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
