Bump @docusaurus/theme-classic from 2.1.0 to 2.3.1 in /website

Bumps [@docusaurus/theme-classic](https://github.com/facebook/docusaurus/tree/HEAD/packages/docusaurus-theme-classic) from 2.1.0 to 2.3.1.
- [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/v2.3.1/packages/docusaurus-theme-classic)

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

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Author: dependabot[bot]