docs: specify script language and add npm install step to installation guide (#13804)

Author: manupik

documentation/docs/01-introduction/02-getting-started.md

@@ -4,9 +4,10 @@ title: Getting started
 
 We recommend using [SvelteKit](../kit), the official application framework from the Svelte team powered by [Vite](https://vite.dev/):
 
-```
+```bash
 npx sv create myapp
 cd myapp
+npm install
 npm run dev
 ```