Add Nextra MVP docs site (GitLab Pages)
Stands up a Next.js + Nextra static site (same stack as spec-unvtd) so the UNSCRO documentation renders on GitLab Pages with its embedded, clickable diagram (GitLab's markdown blob view sanitizes out the /SVG).
- next.config.mjs (static export), mdx-components.js, app/layout.jsx, app/_meta.js, app/page.mdx (landing).
- scripts/prep-site.mjs single-sources the UNSCRO page from vocabulary/unscro.md (rewrites SVG refs, copies the diagram assets into public/); generated files are git-ignored and rebuilt each run.
- .gitlab-ci.yml pages job: npm ci && npm run build (annotate + prep + next build) -> out -> public.
- package.json: Next 15 / Nextra 4.6 / React 18; zod pinned to 4.3.6 via overrides (4.4 broke Nextra's Layout schema; matches spec-unvtd).
This supersedes the interim marked-based Pages job.
Co-Authored-By: Claude Opus 4.8 (1M context) noreply@anthropic.com