feat(v2/seo): per-page last-updated dates + JSON-LD structured data

C — git-revision-date-localized plugin: adds a 'Last update' date to every V2
page (freshness signal for SEO and reader trust). Added the dependency to
requirements.txt, enabled it in v2-mkdocs.yml (type: date,
fallback_to_build_date so the build never fails), and set fetch-depth: 0 on the
06 deploy checkout so the plugin sees full git history (otherwise every page
would show the build date).

D — JSON-LD schema.org structured data: WebSite (with a sitelinks SearchAction)
+ publishing Organization, injected via an extrahead block in docs/overrides/
main.html (shared V1/V2). Enables richer search results / knowledge-panel
eligibility. Validated: build exits 0, JSON-LD parses, 'Last update' renders.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.github/workflows/06.deploy_final.yml

@@ -25,6 +25,9 @@ jobs:
         uses: actions/checkout@v6
         with:
           submodules: true
+          # Full history so mkdocs-git-revision-date-localized can compute the
+          # real per-page "last updated" date (shallow clone => all build-date).
+          fetch-depth: 0
 
       - name: Setup Python
         uses: actions/setup-python@v6