Improve PWA designsystem site features

[azerella]

Bug report

• I’ve read and understood the Contributing guidelines and have done my best effort to follow them.
• I’ve read and agree to the Code of Conduct.
• I’ve searched for any related issues and avoided creating a duplicate issue.

It seems that the current designsystem site isn't very progressive, there's a few things we can look at:

• Add /robots.txt to production environment, this was added for staging/testing with the recent nginx_buildpack addition.
• Add /sitemap.xml this is great for SEO and users looking for a perfect reference of site pages.
• Add service worker to improve asset caching and offline access
• Missing /manifest.json
• Missing <meta description=""/> element
• aria-* attributes do not match their role, see:
  and https://developer.mozilla.org/en-US/docs/Web/Accessibility/ARIA

This issue should be separate into an epic or individual tasks at the least.

[alex-page]

Can we add this to our CI/CD
https://web.dev/fast/using-lighthouse-bot-to-set-a-performance-budget

[azerella]

Closing this as we have other separate issues to address this stuff, e.g #461 #487 #306