feat: multiple playground choices

[slorber]

Motivation

Offer a way to select between CodeSandbox and StackBlitz playgrounds

Persist last choice in a cookie so that user can access his preferred playground faster with the shortcut domains.

[netlify]

✔️ [V2]

🔨 Explore the source changes: 8f2bd6c

🔍 Inspect the deploy log: https://app.netlify.com/sites/docusaurus-2/deploys/60f99aeddbbc7b000bd7179f

😎 Browse the preview: https://deploy-preview-5207--docusaurus-2.netlify.app

[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟠 Performance	88
🟢 Accessibility	98
🟢 Best practices	100
🟢 SEO	100
🟢 PWA	95

Lighthouse ran on https://deploy-preview-5207--docusaurus-2.netlify.app/

[github-actions]

Size Change: +1.34 kB (0%)

Total Size: 852 kB

Filename	Size	Change
website/.docusaurus/globalData.json	59.1 kB	+300 B (+1%)
website/build/assets/js/main.********.js	461 kB	+833 B (0%)
website/build/docs/index.html	40.3 kB	+106 B (0%)
website/build/docs/installation/index.html	47.9 kB	+106 B (0%)

ℹ️ View Unchanged

Filename	Size
website/build/assets/css/styles.********.css	89.1 kB
website/build/blog/2017/12/14/introducing-docusaurus/index.html	62.3 kB
website/build/blog/index.html	26.3 kB
website/build/index.html	27.3 kB
website/build/tests/docs/index.html	20.3 kB
website/build/tests/docs/standalone/index.html	18.8 kB

_{compressed-size-action}

[netlify]

✔️ [V2]

🔨 Explore the source changes: 7ce1f97

🔍 Inspect the deploy log: https://app.netlify.com/sites/docusaurus-2/deploys/60f9bfaef1572a00085c80a0

😎 Browse the preview: https://deploy-preview-5207--docusaurus-2.netlify.app