Add Articles docs section with first use case and samples

Introduce a top-level "Articles" section (docs/articles) that explains UNTP through worked stories, grouped by type. The landing page starts with a "Use cases" table and leaves room for other article types.

First use case, "Publish facility data once, discover it anywhere", follows a manufacturer that issues its DFR via a UNTP software provider and registers the link equally into a facility register, a product registry, and a mapping service, showing DFR/DPP/DIA/DCC and the identity-resolver fan-out.

Add illustrative, non-normative sample glue under artefacts/use-case-samples (facility and product resolver linksets) that reuse the normative samples by link, plus a page template and the "dataspace" cspell entry.

Co-Authored-By: Claude Opus 4.8 (1M context) noreply@anthropic.com