build(deps-dev): bump postcss from 8.2.4 to 8.2.13 in /docs (#984)

Bumps [postcss](https://github.com/postcss/postcss) from 8.2.4 to 8.2.13.
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](https://github.com/postcss/postcss/compare/8.2.4...8.2.13)

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

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

docs/package.json

@@ -20,7 +20,7 @@
     "cross-env": "^7.0.3",
     "mdsvex": "^0.9.8",
     "npm-run-all": "^4.1.5",
-    "postcss": "^8.2.4",
+    "postcss": "^8.2.13",
     "prettier": "^2.5.1",
     "prettier-plugin-svelte": "^2.5.1",
     "prism-svelte": "^0.4.7",