add npm install to quickstart

Lack of it might cause a small stumble to newcomers

Author: kuba-orlik

getting-started/quickstart.md

@@ -23,6 +23,7 @@ sidebar_position: 1
     ```
 3. Build your component with
    ```bash
+   npm install
    npm run build
    ```
 
@@ -32,4 +33,4 @@ Test your component with a [Component Manager](/getting-started/components-manag
 
 ## 💌 Share your Managed Component
 
-Let the community know about your managed component by [making a PR to these docs](https://github.com/managed-components/docs) so it can be shared in the [Known Components](/components) list.
+Let the community know about your managed component by [making a PR to these docs](https://github.com/managed-components/docs) so it can be shared in the [Known Components](/components) list.