docs(docs): improve system design guide and simplify custom page tabs

apps/docs/blume.config.ts

@@ -145,8 +145,8 @@ export default defineConfig({
     // The two custom pages (pages/*.astro) as header tabs. Neither is a
     // content folder, so the tabs scope nothing out of the sidebar tree.
     tabs: [
-      { label: "Changelog", path: "/changelog", icon: "clock" },
-      { label: "Appendix", path: "/appendix", icon: "library" },
+      { label: "Changelog", path: "/changelog" },
+      { label: "Appendix", path: "/appendix" },
     ],
   },
 });