chore(deps-dev): bump docusaurus in /microsite (#2455)

Bumps [docusaurus](https://github.com/facebook/docusaurus) from 2.0.0-alpha.63 to 2.0.0-alpha.64.
- [Release notes](https://github.com/facebook/docusaurus/releases)
- [Changelog](https://github.com/facebook/docusaurus/blob/master/CHANGELOG-2.x.md)
- [Commits](https://github.com/facebook/docusaurus/compare/v2.0.0-alpha.63...v2.0.0-alpha.64)

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

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

microsite/package.json

@@ -13,7 +13,7 @@
     "rename-version": "docusaurus-rename-version"
   },
   "devDependencies": {
-    "docusaurus": "^2.0.0-alpha.61",
+    "docusaurus": "^2.0.0-alpha.64",
     "js-yaml": "^3.14.0"
   }
 }