Improve documentation about headers

Based on discussion in #617 , there is a need for more documentation about headers.

I propose a Header section be added to [Working with Spectrum1Ds](https://specutils.readthedocs.io/en/stable/spectrum1d.html#working-with-spectrum1ds).

Something like:
# Headers
There is currently not universally standard behavior for the handling of header information. In particular, header data is not guaranteed to be preserved or updated. Users should take care with monitoring any important header keywords.

A common practice is to store the primary header in `meta['header']` rather than in `meta` directly.

[a couple examples..]

Open the Working with Spectrum1Ds page linked in the issue and locate its documentation source. Read the discussion in #617 for context, then add the proposed Headers section and a couple of relevant examples. Done means the page clearly explains header preservation and the meta['header'] convention, with the documentation building successfully.

Written by the indexing model from the issue text.