chore: bump @docusaurus/core in /website (#834)

Bumps [@docusaurus/core](https://github.com/facebook/docusaurus/tree/HEAD/packages/docusaurus) from 2.0.0-beta.19 to 2.0.0-beta.20.
- [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.20/packages/docusaurus)

---
updated-dependencies:
- dependency-name: "@docusaurus/core"
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]