build(deps): bump postcss from 8.4.12 to 8.4.13 in /docs

Bumps [postcss](https://github.com/postcss/postcss) from 8.4.12 to 8.4.13.
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](postcss/postcss@8.4.12...8.4.13)

---
updated-dependencies:
- dependency-name: postcss
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>

docs/package.json

@@ -36,8 +36,8 @@
     "gatsby-transformer-remark": "^5.13.0",
     "gatsby-transformer-sharp": "^4.13.0",
     "gatsby-transformer-yaml": "^4.13.0",
-    "polished": "^4.1.4",
-    "postcss": "^8.4.12",
+    "polished": "^4.2.2",
+    "postcss": "^8.4.13",
     "prism-react-renderer": "^1.3.1",
     "prop-types": "^15.8.1",
     "puppeteer": "^13.7.0",