message: docs: update install step in plugin architecture docs
diff: @@ -67,33 +67,41 @@ is available at
  ### Installing plugins
  -Plugins are typically loaded by the UI in your Backstage applications
  -`plugins.ts` file. For example,
  -[here](https://github.com/backstage/backstage/blob/master/packages/app/src/plugins.ts)
  -is that file in the Backstage sample app.
  +Plugins are typically installed as React components in your Backstage
  +application. For example,
  +[here](https://github.com/backstage/backstage/blob/master/packages/app/src/App.tsx)
  +is a file that imports many full-page plugins in the Backstage sample app.
  -Plugins can be enabled, and passed configuration in `apis.ts`. For example,
  -[here](https://github.com/backstage/backstage/blob/master/packages/app/src/apis.ts)
  -is that file in the Backstage sample app.
  -
  -This is how the Lighthouse plugin would be enabled in a typical Backstage
  -application:
  +An example of one of these plugin components is the `CatalogIndexPage`, which is
  +a full-page view that allows you to browse entities in the Backstage catalog. It
  +is installed in the app by importing it and adding it as an element like this:
  ```tsx
  -import { ApiHolder, ApiRegistry } from '@backstage/core';
  -import {
  - lighthouseApiRef,
  - LighthouseRestApi,
  -} from '@backstage/plugin-lighthouse';
  -
  -const builder = ApiRegistry.builder();
  +import { CatalogIndexPage } from '@backstage/plugin-catalog';
  -export const lighthouseApi = new LighthouseRestApi(/* URL of the lighthouse microservice! */);
  -builder.add(lighthouseApiRef, lighthouseApi);
  +...
  -export default builder.build() as ApiHolder;
  +const routes = (
  + <FlatRoutes>
  + ...
  + <Route path="/catalog" element={<CatalogIndexPage />} />
  + ...
  + </FlatRoutes>
  +);
  ```
  +Note that we use `"/catalog"` as our path to this plugin page, but we can choose
  +any route we want for the page, as long as it doesn't collide with the routes
  +that we choose for the other plugins in the app.
  +
  +These components that are exported from plugins are referred to as "Plugin
  +Extension Components", or "Extensions Components". They are regular React
  +components, but in addition to being able to be rendered by React, they also
  +contain various pieces of metadata that is used to write together the entire
  +app. Extensions components are created using `create*Extension` methods, which
  +you can read more about in the
  +[composability documentation](../plugins/composability.md).
  +
  As of this moment, there is no config based install procedure for plugins. Some
  code changes are required.
  