feat(dashboard): collapse Reference nav section by default

Observable Framework 1.13.3 supports collapsible: true on nav sections,
rendering them as <details> elements. Collapsed by default; auto-expands
when any page within the section is active.

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

dashboard/observablehq.config.js

@@ -15,6 +15,7 @@ export default {
     { name: "SBOM", path: "/sbom" },
     {
       name: "Reference",
+      collapsible: true,
       pages: [
         { name: "Live Data", path: "/docs/live-data" },
         { name: "Workstreams", path: "/docs/workstreams" },