Upgrade docusaurus to Upgrade postcss Vulnerability

Summary: Because of the `postcss` vulnerability, I tried to update postcss directly. However, forcing version 8 with previous version of docusaurus threw error. Hence, I upgraded docusaurus instead.

Reviewed By: jknoxville

Differential Revision: D28350050

fbshipit-source-id: ce37a01e3ec817fb006100691bc1d9b1cc33db17

website/package.json

@@ -11,12 +11,12 @@
   },
   "devDependencies": {
     "@babel/parser": "^7.13.12",
-    "@docusaurus/core": "^2.0.0-alpha.72",
-    "@docusaurus/plugin-client-redirects": "2.0.0-alpha.72",
-    "@docusaurus/preset-classic": "^2.0.0-alpha.72",
+    "@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",
     "docblock-parser": "^1.0.0",
-    "docusaurus-plugin-internaldocs-fb": "^0.7.4",
+    "docusaurus-plugin-internaldocs-fb": "^0.8.5",
     "file-cli": "^1.2.0",
     "glob": "^7.1.3",
     "mermaid": "^8.9.2",