docs(tpsc): add GDPR Maturity Model reference page

Full reference for the 7-level CNIL/IAPP CMMI-aligned scale used in TPSC:
source frameworks, per-level descriptions, suitability guidance, key GDPR
concepts (DPA, SCCs, adequacy, BCRs, Art.9), assignment decision tree,
and authoritative references.

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

dashboard/observablehq.config.js

@@ -90,6 +90,7 @@ export default {
         { name: "SCOPE.md",                 path: "/docs/scope" },
         { name: "Tasks",                    path: "/docs/tasks" },
         { name: "TPSC",                     path: "/docs/tpsc" },
+        { name: "TPSC — GDPR Maturity",     path: "/docs/gdpr-maturity" },
         { name: "Technical Debt",           path: "/docs/debt" },
         { name: "Todo",                     path: "/docs/todo" },
         { name: "Workstream Health",        path: "/docs/workstream-health-index" },