Add a windowed sinc to smoothing (for cross-correlation workflow)

This notebook: https://github.com/spacetelescope/jdat_notebooks/blob/c22a9856ac5ce1af947b1f5361b7f425e86625ad/notebooks/redshift_crosscorr/redshift_with_crosscorr.ipynb illustrates using the cross-correlation functionality to get redshifts of galaxies. One issue the `Smooth observation` section reveals: to get the same sorts of results as the IRAF XCORR task one needs to smooth the observations by a windowed sinc function. We should implement that window function in `specutils` itself instead of requiring the user to implement it. Probably the natural thing is to add this window as a function as in that notebook in `specutils/manipulation/smoothing.py` in a way that is compatible with `convolution_smooth`

cc @ibusko as the author of the notebook

Start with the linked redshift_with_crosscorr.ipynb, especially its “Smooth observation” section, to understand the required windowed-sinc behavior. Then inspect specutils/manipulation/smoothing.py and convolution_smooth for the compatible function shape; done means the smoothing option supports the cross-correlation workflow described in the notebook.

Written by the indexing model from the issue text.