chore: Docusaurus with webpack 5 (#3469)

Summary:
Use an unreleased version of Docusaurus to test the webpack5 support works for this site, detect potential problems, and perf improvements.

See also facebook/docusaurus#4089

Perf improvements
- First build (local): 300s
- Second build (local): 90s

To get the benefits, it requires caching `node_modules/.cache`.
It is not possible in GH actions, so I suggest migrating the site to Netlify and use the appropriate toml config (example: https://github.com/react-navigation/react-navigation.github.io/pull/974/files)

 ---

Unrelated suggestion: Relay has a lot of versions and long build/start times

Assuming nobody ever edit Relay docs of 2 years ago, you can improve the DX by limiting the number of versions display in dev with this docs plugin config:

```
          onlyIncludeVersions:
            (isDev || isDeployPreview)
              ? ['current', ...versions.slice(0, 2)]
              : undefined,
```

For faster prod build, you can also move older docs to standalone deployments and link to them (see Jest for versions < 25 => https://jestjs.io/)

Pull Request resolved: #3469

Reviewed By: rbalicki2

Differential Revision: D28127629

Pulled By: jstejada

fbshipit-source-id: c78309aa1c7f14f22ecc33166ad5dce7d2e51f94

Author: slorber

website/package.json

@@ -18,19 +18,19 @@
     "docusaurus": "docusaurus"
   },
   "dependencies": {
-    "@docusaurus/core": "2.0.0-alpha.70",
-    "@docusaurus/plugin-client-redirects": "2.0.0-alpha.70",
-    "@docusaurus/preset-classic": "2.0.0-alpha.70",
+    "@docusaurus/core": "2.0.0-alpha.75",
+    "@docusaurus/plugin-client-redirects": "2.0.0-alpha.75",
+    "@docusaurus/preset-classic": "2.0.0-alpha.75",
     "classnames": "^2.2.6",
     "clsx": "^1.1.1",
-    "docusaurus-plugin-internaldocs-fb": "^0.7.0",
-    "react": "^16.10.2",
-    "react-dom": "^16.10.2",
+    "docusaurus-plugin-internaldocs-fb": "^0.8.5",
+    "react": "^17.0.2",
+    "react-dom": "^17.0.2",
     "react-syntax-highlighter": "15.3.1",
     "spec-md": "^1.1.1"
   },
   "devDependencies": {
-    "docusaurus": "^2.0.0-alpha.70"
+    "docusaurus": "2.0.0-alpha.378053ac5"
   },
   "resolutions": {
     "highlight.js": "^10.4.1"