chore: Upgrade Docusaurus

[c-warren]

Upgrade docusaurus to 3.9.1

Snyk is desperately trying to upgrade docusaurus for us:

• [Snyk] Fix for 4 vulnerabilities #284
• [Snyk] Upgrade @docusaurus/plugin-content-pages from 3.7.0 to 3.8.1 #279
• [Snyk] Upgrade @docusaurus/plugin-content-docs from 3.7.0 to 3.8.1 #278
• [Snyk] Upgrade @docusaurus/plugin-content-blog from 3.7.0 to 3.8.1 #277
• [Snyk] Upgrade @docusaurus/plugin-client-redirects from 3.7.0 to 3.8.1 #276
• [Snyk] Upgrade @docusaurus/core from 3.7.0 to 3.8.1 #275

But docusaurus requires that all packages be of the same type, and Snyk is trying to do it one by one which is causing the builds to fail.

[Bueller87]

Regarding the failing build workflow: workflows/build.yml

• Is cache: 'npm' necessary? I might try the build again without it
• it's generally a better practice to use 'npm ci' (clean install) over 'npm install' in automated builds
  e.g.
  https://github.com/cadence-workflow/cadence-web/blob/master/.github/workflows/build.yml