docs(docs): add new problem documentation pages and source selector component

apps/docs/components.ts

@@ -16,5 +16,8 @@ export default defineComponents({
   layout: {
     // Topic landing pages' "what links here": every problem pointing at the page.
     PageFooter: "./components/BacklinksList.astro",
+    // Content-source selector + pinned links above the nav tree, scoped to the
+    // active source (sources.ts).
+    Sidebar: "./components/SourceSidebar.astro",
   },
 });