Updating documentation  and testing

This is starting to update the specutils 1D documentation and testing to show this a more broad audience. 

These are the points that I think need to do:

- [x] update docstrings
- [x] update read_fits testing
- [x] update documentation for general `Spectrum1D` usage
- [x] write documentation for `specwcs` with explanations of structure
- [ ] write test examples for constructing custom WCS (@keflavich suggestions welcome here)
- [x] write documentation for reading FITS files

@keflavich @mhvk please add to the list if you think there's anything missing.