Migrate UNTP Specification Website to untp.unece.org Domain

The UNICC team has enabled support for custom subdomains for project websites. To align with UNECE branding and improve accessibility, the UNTP Specification Website will transition from https://spec-untp-fbb45f.opensource.unicc.org/ to https://untp.unece.org.

The migration will involve coordination with UNECE IT to configure the required DNS records, followed by updates across the website, documentation, and repository references. The existing transition message on the website and in the README (GitHub) will also be updated to reflect the new URL.

Acceptance Criteria:

• Coordinate with UNECE IT to enable the new untp.unece.org domain.
• Verify that the new domain resolves correctly.
• Update the existing transition message on the website (GitHub) to reference https://untp.unece.org.
• Update the transition notice within the repository README (GitHub) to reference the new domain.
• Update all internal links within the UNTP specification website to reference the new domain.
• Update documentation pages and assets referencing the old domain.
• Update repository descriptions, issues, merge requests, and comments that contain the old domain.
• Communicate the domain change to the broader UNTP community once complete (Mailing List and Slack Channel).