Unpin Docusaurus from 3.8.1 canary and upgrade to ^3.9.0 (meta-pytorch#3258)

Summary:
- Removes the pinned `3.8.1-canary-6401` Docusaurus version and the associated TODO comment (T239136996)
- Upgrades all Docusaurus packages to `^3.9.0` (resolves to 3.9.2)
- Migrates deprecated `siteConfig.onBrokenMarkdownLinks` to `siteConfig.markdown.hooks.onBrokenMarkdownLinks` per Docusaurus v4 deprecation warning
- Regenerates `yarn.lock`

Pull Request resolved: meta-pytorch#3258

Test Plan:
- [ ] Verify website builds locally with `bash scripts/build_docs.sh -b`
- [ ] Verify website renders correctly with `bash scripts/build_docs.sh`
<img width="2555" height="1350" alt="Screenshot 2026-04-01 at 3 42 29 PM" src="https://github.com/user-attachments/assets/ff00ecc6-81bb-4edd-b679-e3d2893c09fd" />
<img width="2541" height="1347" alt="Screenshot 2026-04-01 at 3 42 22 PM" src="https://github.com/user-attachments/assets/cecb1722-6fef-493f-995b-120a584eb7f8" />

Reviewed By: esantorella

Differential Revision: D99145378

Pulled By: saitcakmak

fbshipit-source-id: b63a51182bb7ab4010b35b81f473acde1ccd5518

Author: saitcakmak

website/docusaurus.config.js

@@ -21,7 +21,10 @@ module.exports={
     "https://cdnjs.cloudflare.com/ajax/libs/clipboard.js/2.0.0/clipboard.min.js",
   ],
   "markdown": {
-    format: "detect"
+    format: "detect",
+    hooks: {
+      onBrokenMarkdownLinks: "warn"
+    }
   },
   "stylesheets": [
     "/css/code_block_buttons.css",
@@ -39,7 +42,6 @@ module.exports={
     "wrapPagesHTML": true
   },
   "onBrokenLinks": "throw",
-  "onBrokenMarkdownLinks": "warn",
   "presets": [
     [
       "@docusaurus/preset-classic",

website/package.json

@@ -11,22 +11,21 @@
     "write-heading-ids": "docusaurus write-heading-ids"
   },
   "devDependencies": {
-    "@docusaurus/module-type-aliases": "3.8.1-canary-6401",
-    "@docusaurus/types": "3.8.1-canary-6401"
+    "@docusaurus/module-type-aliases": "^3.9.0",
+    "@docusaurus/types": "^3.9.0"
   },
-  "//": "TODO: T239136996 Unpin Docusaurus canary after 3.8.2 or 3.9 is released",
   "dependencies": {
-    "@docusaurus/core": "^3.8.1-canary-6401",
-    "@docusaurus/preset-classic": "^3.8.1-canary-6401",
+    "@docusaurus/core": "^3.9.0",
+    "@docusaurus/preset-classic": "^3.9.0",
     "bl": "^6.0.0",
     "clsx": "^1.1.1",
+    "on-headers": "^1.1.0",
     "plotly.js": "^2.8.1",
     "react": "^18.3.1",
     "react-dom": "^18.3.1",
     "react-plotly.js": "^2.5.1",
     "rehype-katex": "7",
     "remark-math": "6",
-    "on-headers": "^1.1.0",
     "webpack-dev-server": "^5.2.1"
   }
 }