gen-art-51

- Added anchor to Probe Request Definition section.
- Added links to registry definitions for:
  - Overlay Algorithm Type
  - Application-ID
  - Data Model
  - Error Code
  - Overlay Link Registry
  - Link Protocols
  - Forwarding Option
  - Probe Information Types
  - Message Extensions
Merge pull request #178 from petithug/gen-art-51

Gen-ART 51 closes Issue 178 via cleanup of the IANA registry section.