feat(customization): support customizing catalog and api index pages via outlets

Signed-off-by: Phil Kuang <pkuang@factset.com>

docs/features/software-catalog/catalog-customization.md

@@ -10,8 +10,7 @@ and find catalog entities. This is already set up by default by
 `@backstage/create-app`.
 
 If you want to change the default index page - such as to add a custom filter to
-the catalog - you can replace the routing in `App.tsx` to point to your own
-`CatalogIndexPage`.
+the catalog - you can create your own `CatalogIndexPage`.
 
 > Note: The catalog index page is designed to have a minimal code footprint to
 > support easy customization, but creating a copy does introduce a possibility
@@ -21,12 +20,11 @@ the catalog - you can replace the routing in `App.tsx` to point to your own
 
 For example, suppose that I want to allow filtering by a custom annotation added
 to entities, `company.com/security-tier`. To start, I'll copy the code for the
-default catalog page and create a component in a
-[new plugin](../../plugins/create-a-plugin.md):
+default catalog page and create a component.
 
 ```tsx
 // imports, etc omitted for brevity. for full source see:
-// https://github.com/backstage/backstage/blob/master/plugins/catalog/src/components/CatalogPage/CatalogPage.tsx
+// https://github.com/backstage/backstage/blob/master/plugins/catalog/src/components/CatalogPage/DefaultCatalogPage.tsx
 export const CustomCatalogPage = ({
   columns,
   actions,
@@ -167,21 +165,7 @@ export const CustomCatalogPage = ({
 };
 ```
 
-This page itself can be exported as a routable extension in the plugin:
-
-```ts
-export const CustomCatalogIndexPage = myPlugin.provide(
-  createRoutableExtension({
-    name: 'CustomCatalogIndexPage',
-    component: () =>
-      import('./components/CustomCatalogPage').then(m => m.CustomCatalogPage),
-    mountPoint: catalogRouteRef,
-  }),
-);
-```
-
-Finally, we can replace the catalog route in the Backstage application with our
-new `CustomCatalogIndexPage`.
+Finally, we can apply our new `CustomCatalogPage`.
 
 ```diff
 # packages/app/src/App.tsx
@@ -189,7 +173,9 @@ const routes = (
   <FlatRoutes>
     <Navigate key="/" to="catalog" />
 -    <Route path="/catalog" element={<CatalogIndexPage />} />
-+    <Route path="/catalog" element={<CustomCatalogIndexPage />} />
++    <Route path="/catalog" element={<CatalogIndexPage />}>
++      <CustomCatalogPage />
++    </Route>
 ```
 
 The same method can be used to customize the _default_ filters with a different