Bump postcss and autoprefixer in /docs (#5114)

Bumps [postcss](https://github.com/postcss/postcss) to 8.4.40 and updates ancestor dependency [autoprefixer](https://github.com/postcss/autoprefixer). These dependencies need to be updated together.

Updates `postcss` from 7.0.39 to 8.4.40
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](postcss/postcss@7.0.39...8.4.40)

Updates `autoprefixer` from 9.8.8 to 10.4.20
- [Release notes](https://github.com/postcss/autoprefixer/releases)
- [Changelog](https://github.com/postcss/autoprefixer/blob/main/CHANGELOG.md)
- [Commits](postcss/autoprefixer@9.8.8...10.4.20)

---
updated-dependencies:
- dependency-name: postcss
  dependency-type: indirect
- dependency-name: autoprefixer
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Signed-off-by: pipecd-bot <[email protected]>

docs/package.json

@@ -17,7 +17,7 @@
   },
   "homepage": "https://github.com/google/docsy-example#readme",
   "devDependencies": {
-    "autoprefixer": "^9.8.6",
+    "autoprefixer": "^10.4.20",
     "postcss": "^8.4.31",
     "postcss-cli": "^8.3.1"
   }