Restructure ADIA specs

[rlin1]

Goal:
a) make it easier for platform vendors to implement platform level support
b) make it easier for SPs and Issuer to read and implement.

[rlin1]

First idea for structure: see https://docs.google.com/document/d/11ClTipppB_iM5QcsBQ2iRzmnSVUr4pYQgmhl7g8Dykk/edit#