docs(dashboard): add technical reference page for Observable Framework dashboard

Documents the dashboard's architecture, framework choice rationale, data-fetching
strategies (static loaders + live polling), component library, page inventory,
and key features including the Workstream Health Index and entity modals.
Also registers the new page in the Reference nav and adds runbook section for
node overload / runaway agent process (INC-002) with hardening checklist.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

state-hub/dashboard/observablehq.config.js

@@ -72,6 +72,7 @@ export default {
       pages: [
         { name: "Capabilities",              path: "/docs/capabilities" },
         { name: "Connecting to the Hub",    path: "/docs/connecting" },
+        { name: "Dashboard",               path: "/docs/dashboard" },
         { name: "Contributions",            path: "/docs/contributions" },
         { name: "Decision Health",          path: "/docs/decisions-kpi" },
         { name: "Decisions",               path: "/docs/decisions" },