chore(deps-dev): bump @docusaurus/theme-common

Bumps [@docusaurus/theme-common](https://github.com/facebook/docusaurus/tree/HEAD/packages/docusaurus-theme-common) from 2.0.0-beta.20 to 2.0.0-beta.21.
- [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/v2.0.0-beta.21/packages/docusaurus-theme-common)

---
updated-dependencies:
- dependency-name: "@docusaurus/theme-common"
  dependency-type: direct:development
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>