Expand the root spec document

[afrittoli]

The root specification document spec.md includes aspects of CDEvents which are common across all events.
We shall restructure it and expand it a bit, following the example from CloudEvents: https://github.com/cloudevents/spec/blob/v1.0.1/spec.md#notations-and-terminology:

• Overview
• Version (we can start with 0.1)
• Notation and Terminology
• Common attributes: for each attribute include type, description, examples, use cases
• Link to the spec for each event bucket