chore: bump @docusaurus/theme-common from 3.8.1 to 3.9.1 in /site

Bumps [@docusaurus/theme-common](https://github.com/facebook/docusaurus/tree/HEAD/packages/docusaurus-theme-common) from 3.8.1 to 3.9.1.
- [Release notes](https://github.com/facebook/docusaurus/releases)
- [Changelog](https://github.com/facebook/docusaurus/blob/main/CHANGELOG.md)
- [Commits](https://github.com/facebook/docusaurus/commits/v3.9.1/packages/docusaurus-theme-common)

---
updated-dependencies:
- dependency-name: "@docusaurus/theme-common"
  dependency-version: 3.9.1
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]