build(deps): bump postcss from 8.4.18 to 8.4.20 in /docs (#3417)

Author: dependabot[bot]

docs/package-lock.json

@@ -37,7 +37,7 @@
     "gatsby-transformer-sharp": "^4.21.0",
     "gatsby-transformer-yaml": "^4.21.0",
     "polished": "^4.2.2",
-    "postcss": "^8.4.18",
+    "postcss": "^8.4.20",
     "prism-react-renderer": "^1.3.5",
     "prop-types": "^15.8.1",
     "puppeteer": "^13.7.0",