Upgrade Docusaurus from pinned canary to ^3.9.0 (facebook#5121)

Summary:
- Unpin `docusaurus/*` packages from `3.8.1-canary-6401` to `^3.9.0` (resolves T239136996)
- Remove the TODO comment about unpinning
- Migrate deprecated top-level `onBrokenMarkdownLinks` config to `markdown.hooks.onBrokenMarkdownLinks` for Docusaurus v4 compatibility

Pull Request resolved: facebook#5121

Test Plan:
- [ ] `cd website && yarn install && yarn build` succeeds
- [ ] Website renders correctly with `yarn start`
<img width="2519" height="1347" alt="Screenshot 2026-04-01 at 3 51 09 PM" src="https://github.com/user-attachments/assets/478c4ced-edf0-4485-9d93-b7fa2a9d57dc" />
<img width="2550" height="1332" alt="Screenshot 2026-04-01 at 3 51 27 PM" src="https://github.com/user-attachments/assets/73e23928-41d2-4de7-8057-210a00b87f70" />
<img width="2534" height="1341" alt="Screenshot 2026-04-01 at 3 51 20 PM" src="https://github.com/user-attachments/assets/ec7941ce-06d3-44d7-9b4b-06d68814693b" />

Reviewed By: esantorella

Differential Revision: D99146599

Pulled By: saitcakmak

fbshipit-source-id: f82ffbe139ddfbc7da7816332be7872b4f43cb17

Author: saitcakmak

website/docusaurus.config.js

@@ -29,7 +29,11 @@ module.exports={
     "wrapPagesHTML": true
   },
   "onBrokenLinks": "throw",
-  "onBrokenMarkdownLinks": "warn",
+  markdown: {
+    hooks: {
+      onBrokenMarkdownLinks: "warn",
+    },
+  },
   "presets": [
     [
       "@docusaurus/preset-classic",

website/package.json

@@ -1,4 +1,5 @@
 {
+  "license": "MIT",
   "scripts": {
     "examples": "docusaurus-examples",
     "start": "docusaurus start",
@@ -14,11 +15,10 @@
     "docusaurus": "docusaurus"
   },
   "devDependencies": {},
-  "//": "TODO: T239136996 Unpin Docusaurus canary after 3.8.2 or 3.9 is released",
   "dependencies": {
-    "@docusaurus/core": "3.8.1-canary-6401",
-    "@docusaurus/plugin-client-redirects": "3.8.1-canary-6401",
-    "@docusaurus/preset-classic": "3.8.1-canary-6401",
+    "@docusaurus/core": "^3.9.0",
+    "@docusaurus/plugin-client-redirects": "^3.9.0",
+    "@docusaurus/preset-classic": "^3.9.0",
     "clsx": "^1.1.1",
     "plotly.js": "^2.8.1",
     "react": "^18.3.1",