docs: update dte spec to v0.6.0

Created by: ashleythedeveloper

This PR updates the DTE specification page to align with the v0.6.0 data model.

NOTE: External example DTEs (links, QR codes) will be updated at a later date.

The changes include:

• Update external inks to point to v0.6.0 artefacts (DTE 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