docs: add npm install command

`npm run dev` will give an error if you don't first run `npm install`

Author: manupik

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

@@ -7,6 +7,7 @@ We recommend using [SvelteKit](../kit), the official application framework from
 ```bash
 npx sv create myapp
 cd myapp
+npm install
 npm run dev
 ```