Bump postcss from 8.2.9 to 8.4.31 in /docs (#654)

Bumps [postcss](https://github.com/postcss/postcss) from 8.2.9 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@8.2.9...8.4.31)

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

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

docs/package.json

@@ -1,7 +1,7 @@
 {
   "dependencies": {
     "postcss-cli": "8.3.1",
-    "postcss": "8.2.9",
+    "postcss": "8.4.31",
     "autoprefixer": "10.2.5"
   }
 }