Bump postcss from 8.4.18 to 8.4.31 in /docs (#2752)

Bumps [postcss](https://github.com/postcss/postcss) from 8.4.18 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@8.4.18...8.4.31)

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

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

docs/package.json

@@ -16,7 +16,7 @@
     "@types/markdown-it": "^12.2.3",
     "@types/node": "^16.11.65",
     "vitepress": "1.0.0-alpha.21",
-    "postcss": "^8.4.18",
+    "postcss": "^8.4.31",
     "postcss-import": "^14.1.0",
     "terser": "^5.15.1"
   },