upgrade docusaurus dependency

[pixelb]

Note the following does not update to the latest beta docusaurus
$ yarn upgrade docusaurus docusaurus-plugin-internaldocs-fb --latest
Therefore manually set "2.0.0-beta.15" package.json and rerun yarn upgrade

The docusaurus.config.js change is required in the new version due to:
facebook/docusaurus#5832

The website/src/components/GithubLink.jsx change is required due to:
facebook/docusaurus#6516

Motivation

(Write your motivation for proposed changes here.)

Have you read the Contributing Guidelines on pull requests?

Yes/No

Test Plan

(How should this PR be tested? Do you require special setup to run the test or repro the fixed bug?)

Related Issues and PRs

(Is this PR part of a group of changes? Link the other relevant PRs and Issues here. Use https://help.github.com/en/articles/closing-issues-using-keywords for help on GitHub syntax)

[pixelb]

This needs a change in docusaurus-plugin-internaldocs-fb also due to facebook/docusaurus#6516,
so we need to wait for that. I've asked the docusaurus-plugin-internaldocs-fb author how best to proceed with that

[facebook-github-bot]

Hi @pixelb!

Thank you for your pull request.

We require contributors to sign our Contributor License Agreement, and yours needs attention.

You currently have a record in our system, but the CLA is no longer valid, and will need to be resubmitted.

Process

In order for us to review and merge your suggested changes, please sign at https://code.facebook.com/cla. If you are contributing on behalf of someone else (eg your employer), the individual CLA may not be sufficient and your employer may need to sign the corporate CLA.

Once the CLA is signed, our tooling will perform checks and validations. Afterwards, the pull request will be tagged with CLA signed. The tagging process may take up to 1 hour after signing. Please give it that time before contacting us about it.

If you have received this in error or have any questions, please contact us at cla@meta.com. Thanks!