Add documentation for Exporter plugins
Ticket moved to GitHub: "pulp/pulpcore/1814":https://github.com/pulp/pulpcore/issues/1814
Exporters should be documented separately from the other objects because in many cases, they will exist in their own isolated plugin.
For example a filesystem exporter would need to be its own plugin because writing publications to a filesystem is a general need and can be done for any content type.
This documentation should explain how to implement the Exporter Model, Viewset, and Serializer. It should also explain how to use the plugin template to create an Exporter plugin. (The template will include unnecessary objects, like an Importer.)