refactor(web): clean up components in beta

web/src/beta/README.md

@@ -0,0 +1,9 @@
+# Component Structure
+
+```planttext
+pages                             // The main pages of the app.
+└──features                       // Features of the app, used in pages. Handles business logic.
+    ├──ui                         // Pure UI, Re:Earth Visualizer specific UI components.
+    │   └──lib/reearth-ui         // Pure UI, Re:Earth common UI components.
+    └──lib/reearth-ui
+```