IDR specification registry identifiers vs self-issued identifiers.
This PR refactors the Identity Resolver specification, by adding no new meaning content but restructuring existing content.
- Reorganised document structure. Registry-managed and self-issued identifiers are the two leading sections of the document. Sections underneath follow D-R-V structure, with shared references and requirements at the end of the document.
 - Moved technical specifics to the end of the document for better flow.
 - Added cross-references between related sections for better navigation.
 - Removed discussion around verification which is out of scope for this page, and required in specification about Trust/Transparency Graphs.
 - Practical real world examples for both self-issued and registry managed identifiers
 - Respond to issue and comments raised in #347
 
Edited  by Harley Thomas