Add docs section addressing SpectralCube transition

Now that we're transitioning functionality from `spectral-cube` into `specutils` (e.g. #753, #755, #758), there should be a section in the docs that explains these functions and any differences between the way they were implemented in `spectral-cube` (e.g. the fact that they're `manipulation` functions rather than class methods, different upper bound handling in `spectral_slab`).

Review the existing specutils documentation and the changes described in issues #753, #755, and #758 first. Add a section covering the functionality transitioned from spectral-cube, including the manipulation-function API and the spectral_slab upper-bound difference; the documentation is done when these transition details and differences are clearly explained.

Written by the indexing model from the issue text.