docs: update plugin installation docs

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>

docs/features/kubernetes/installation.md

@@ -23,16 +23,8 @@ cd packages/app
 yarn add @backstage/plugin-kubernetes
 ```
 
-Once the package has been installed, you need to import the plugin in your app.
-Add the following to `packages/app/src/plugins.ts`:
-
-`plugins.ts`:
-
-```typescript
-export { plugin as Kubernetes } from '@backstage/plugin-kubernetes';
-```
-
-Now, add the "Kubernetes" tab to the catalog entity page. In
+Once the package has been installed, you need to import the plugin in your app
+by adding the "Kubernetes" tab to the catalog entity page. In
 `packages/app/src/components/catalog/EntityPage.tsx`, you'll add a router to get
 to the tab, and add the tab itself.
 

docs/features/software-catalog/installation.md

@@ -24,14 +24,8 @@ yarn add @backstage/plugin-catalog
 
 ### Adding the Plugin to your `packages/app`
 
-Add the following entry to the head of your `packages/app/src/plugins.ts`:
-
-```ts
-export { catalogPlugin } from '@backstage/plugin-catalog';
-```
-
-Next we need to install the two pages that the catalog plugin provides. You can
-choose any name for these routes, but we recommend the following:
+Add the two pages that the catalog plugin provides to your app. You can choose
+any name for these routes, but we recommend the following:
 
 ```tsx
 // packages/app/src/App.tsx

docs/features/software-templates/installation.md

@@ -26,14 +26,8 @@ yarn add @backstage/plugin-scaffolder
 
 ### Adding the Plugin to your `packages/app`
 
-Add the following entry to the head of your `packages/app/src/plugins.ts`:
-
-```ts
-export { scaffolderPlugin } from '@backstage/plugin-scaffolder';
-```
-
-Next we need to install the root page that the Scaffolder plugin provides. You
-can choose any path for the route, but we recommend the following:
+Add the root page that the Scaffolder plugin provides to your app. You can
+choose any path for the route, but we recommend the following:
 
 ```tsx
 import { ScaffolderPage } from '@backstage/plugin-scaffolder';

docs/features/techdocs/getting-started.md

@@ -29,14 +29,8 @@ yarn add @backstage/plugin-techdocs
 
 Once the package has been installed, you need to import the plugin in your app.
 
-Add the following to `packages/app/src/plugins.ts`:
-
-```typescript
-export { plugin as TechDocs } from '@backstage/plugin-techdocs';
-```
-
-Now we can add a route for the TechDocs page. In `packages/app/src/App.tsx`,
-import TechDocsPage and add the following to `FlatRoutes`:
+In `packages/app/src/App.tsx`, import TechDocsPage and add the following to
+`FlatRoutes`:
 
 ```tsx
 import { TechDocsPage } from '@backstage/plugin-techdocs';

docs/getting-started/configure-app-with-plugins.md

@@ -27,14 +27,7 @@ package.json. Backstage Apps are set up as monorepos with
 [yarn workspaces](https://classic.yarnpkg.com/en/docs/workspaces/). Since
 CircleCI is a frontend UI plugin, it goes in `app` rather than `backend`.
 
-2. Add the plugin itself to the App:
-
-```js
-// packages/app/src/plugins.ts
-export { plugin as CircleCi } from '@backstage/plugin-circleci';
-```
-
-3. Register the plugin in the entity pages:
+2. Add the `EntityCircleCIContent` extension to the entity pages in the app:
 
 ```diff
  // packages/app/src/components/catalog/EntityPage.tsx

docs/plugins/structure-of-a-plugin.md

@@ -100,8 +100,7 @@ There are three things needed for a Backstage app to start making use of a
 plugin.
 
 1. Add plugin as dependency in `app/package.json`
-2. `import` plugin in `app/src/plugins.ts`
-3. Import and use one or more plugin extensions, for example in
+2. Import and use one or more plugin extensions, for example in
    `app/src/App.tsx`.
 
 Luckily these three steps happen automatically when you create a plugin with the