OC-2372: added download script in self-hosting-example

[wenjunche]

1. to clean public folder: npm run build-clean
2. to download RVM and stable Runtime, and create public/app.json with stable version of Runtime: npm run build
3. to download multiple versions of Runtime (not updating app.json):
   npm run build -- --runtimes=canary,28.106.72.14,stable,26.102.71.10,22.94.67.5