docs: update dcc spec to v0.6.0
Created by: ashleythedeveloper
This PR updates the DCC specification page to align with the v0.6.0 data model.
NOTE: The external example DCC (link, QR code) will be updated at a later date.
The changes include:
- Update external inks to point to v0.6.0 artefacts (DCC and UNTP Core)
- Update the image of the data model
- Update property names to align with v0.6.0
- Update JSON examples to align with v0.6.0
- Tweaks to the descriptions
- Removed the section that stated the DCC and DPP use the Declaration structure (now ConformityAttestation for DCC) as the DPP data model doesn't use the Declaration or ConformityAttestation structure but instead uses Claim. I assume this will be revised in 0.7.0.