docs: add v0.7.0 sample credential links and QR codes to spec pages

This MR adds a Sample Credential section to each WIP credential specification page (DCC, DFR, DIA, DPP, DTE) in preparation for the v0.7.0 release.

Each section includes a URL and a QR code linking to a signed v0.7.0 credential, allowing readers to preview a sample of the credential.