Re: Minimal WPUB for a scholarly paper (of sort)

While all of these proposed solution work fine from a purely JSON-LD/RDF
perspective, they're IMO a little too smart for their own good.

If we don't plan on using schema.org for the "default reading order" and
the "list of resources" terms, we should define them in our own context
file instead.

I would much rather have:

"@context": ["https://schema.org", "https://www.w3.org/TR/wpub/"]

instead of:

"@context": [
    "https://schema.org",
    {
      "publ-resources" : null
    }
  ]

IMO this shouldn't be an issue with how Google and other JSON-LD clients
will extract info from our manifest, they'll simply ignore the terms that
they don't know about.

There are other benefits as well to using our own context document:

   - it's a much cleaner framework to extend our manifest than mapping
   individual elements in @context
   - we want to map some terms to bib.schema.org as well (various
   contributor roles, cover)
   - we might also need to map some terms to EPUB 3.2

Hadrien