Move more properties to the "Glossary"; rename to "Core Concepts"?

[roll]

Overview

As a consequence of the organic growth of the Standard usage we got a situation where we reference some properties from other specs e.g.:

• resource.contributors -> package.contributors
• Promote "Metadata in Table Schema" recipe to the specs. datapackage-v2-draft#63
• etc

We just need to move common ones into the Glossary and reference them from there

[nichtich]

The formal role of the glossary should be made more clear. Is it documentation to explain basic concepts? Is it specification with formal definition of data elements? For the latter the term "glossary" should better be replaced by some other name, e.g. "Core Concepts".

[roll]

@nichtich
The "Glossary" is intended to be a spec as it defines building blocks for other specs that requires to be on the same level of formality (e.g. regarding changes etc)

I agrees that "Core Concepts" or similar might be a better name