Add Google Analytics gtag to Cadence Lang

[flow-slack-issue-bot]

Reported by: Ali Serag (<@ali.serag>)

Issue Description:

Add google analytics tracking to Cadence Lang website so that all pages are tracked (I believe we'd have to add the env variable in vercel and update the docusaurus.config file similar to how we did for the regular dev docs site. The following is the generated tag:

<!-- Google tag (gtag.js) -->
<script async src="https://www.googletagmanager.com/gtag/js?id=G-5654FKLB20"></script>
<script>
  window.dataLayer = window.dataLayer || [];
  function gtag(){dataLayer.push(arguments);}
  gtag('js', new Date());

  gtag('config', 'G-5654FKLB20');
</script>