Solid OIDC Introduction to clarify goals and proposed functionality

[csarven]

s/utilizing/using

Add more detail on what the implementer wants to achieve, i.e., to enable who and to do what.

Consider changing last paragraphs starting "The OAuth 2.0 [RFC6749]" to explain what they offer and why/how they are of use to the Solid ecosystem, what they're missing and how this spec extends with "additional functionality".