Bump @docusaurus/core from 2.0.0-alpha.71 to 2.0.0-beta.0 in /website (…

…#1307)

Bumps [@docusaurus/core](https://github.com/facebook/docusaurus/tree/HEAD/packages/docusaurus) from 2.0.0-alpha.71 to 2.0.0-beta.0.
- [Release notes](https://github.com/facebook/docusaurus/releases)
- [Changelog](https://github.com/facebook/docusaurus/blob/master/CHANGELOG.md)
- [Commits](https://github.com/facebook/docusaurus/commits/v2.0.0-beta.0/packages/docusaurus)

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