Update X-ARF spec according to meeting notes with Tobias

* Updated "idea" description of X-ARF.
* Define a "plain" report as MIME headers followed by the report body parts
* Reordered sections
* Transport is now separate from format; an X-ARF transport should also define security and bulk format.
* Update URIs as identities to be explanatory rather than normative; the schema will specify details of URIs
* Updated overview to remove header sections - those should be described in a transport document
* Removed appendix A, since secure and bulk should be defined in a transport document
* Updated appendix B to be a description of common YAML keys and semantics; report type schemata and guides should implement these
* Renamed Appendix C to A, and updated it to illustrate only the "plain" X-ARF report

Also, worked the SMTP-related specs removed from X-ARF proper into the
xarf-smtp-specification.