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

apps/docs/blume.config.ts

@@ -142,11 +142,8 @@ export default defineConfig({
     sidebar: {
       display: "flat", // "flat" | "group" | "page"
     },
-    // The glossary is no content folder any more, so it is pinned above the
-    // sidebar tree, on every page and breakpoint.
-    featured: [
-      { label: "Changelog", href: "/changelog", icon: "clock" },
-      { label: "Appendix", href: "/appendix", icon: "library" },
-    ],
+    // No `featured`: Blume renders it above the `Sidebar` slot, and the
+    // content-source selector must sit above the pinned links, so
+    // components/SourceSidebar.astro renders both (Changelog, Appendix).
   },
 });