DOC: There is insufficient documentation on writers

At the time of posting this, `.write` is not mentioned at all in https://specutils.readthedocs.io/en/latest/spectrum1d.html except as a column in [the table](https://specutils.readthedocs.io/en/latest/spectrum1d.html#list-of-loaders), which mostly says "no" for most of the supported formats.

The API documentation at https://specutils.readthedocs.io/en/latest/api/specutils.Spectrum1D.html#specutils.Spectrum1D.write is also very confusing because it keeps referring to `CCDData`.

Start with the spectrum1d.html page and the Spectrum1D.write API documentation linked in the issue, comparing the writer information with the existing loader table. Clarify how .write is used and correct the confusing CCDData references; the work is done when the documentation accurately describes writers and supported formats.

Written by the indexing model from the issue text.