Clarify publication and rendition

[mattgarrish]

We should add an introductory section that clarifies what we mean when we distinguish epub publication from rendition and then talk about the two sometimes interchangeably in the metadata.

A title for the publication may not be different from how it is expressed in a rendition. Same for unique identifiers or anything else. Or differences may only be in language, contributors (translators), etc.

Each rendition is an expression of a publication, but it is still in every meaningful sense the publication. Having to read the definitions alone to glean this isn't optimal.

I'm just not sure yet which document in belongs in, but leaning to an elaboration in packages since the new core spec will have the roadmap for some distinction there.

[mattgarrish]

Noting that we also have "packages".

Publication conceptually covers everything in the Container, and a Rendition would be the embodiment of a Package (Package Document + resources).

Might be useful to create a graphic to show these relations.

[mattgarrish]

I opened this before we decided not to incorporate the satellite specs. I'm closing off as rejected, as any further elaboration requires the details to be solved.