build(deps-dev): bump postcss from 8.4.8 to 8.4.31 in /site

Bumps [postcss](https://github.com/postcss/postcss) from 8.4.8 to 8.4.31.
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](postcss/postcss@8.4.8...8.4.31)

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

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

site/package-lock.json

@@ -18,7 +18,7 @@
   "homepage": "https://github.com/google/docsy-example#readme",
   "devDependencies": {
     "autoprefixer": "^10.4.2",
-    "postcss": "^8.4.8",
+    "postcss": "^8.4.31",
     "postcss-cli": "^9.1.0"
   }
 }