Rework to include schema.org types

[nickevansuk]

This reworks the spec to include schema.org types, and reframes it as a series of schema.org extension proposals, to satisfy #6:

• Takes into account the new DCAT v2, which has sought feedback from the schema.org community on alignment regarding similar concerns.

• Removed the notion of "array", "required" and "optional", to bring it in line with schema.org's modelling approach.

• Refactored into modular concrete schema.org proposals with discrete rationale, using the requisite format for such properties in schema.org. This hopefully allows us to push forward at least some of these in a reasonable timeframe.

• Added a scope section to make the intention clear

@MikeRalphson if you like the structure, I can add a GitHub issue for each mini-proposal to encourage feedback on each, ahead of it being shared more widely? Let me know what you think!

[nickevansuk]

No problem at all! Perfect, have made the changes suggested.