Bump postcss from 7.0.39 to 8.4.40 in /docs (#5099)

Bumps [postcss](https://github.com/postcss/postcss) from 7.0.39 to 8.4.40.
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](postcss/postcss@7.0.39...8.4.40)

---
updated-dependencies:
- dependency-name: postcss
  dependency-type: indirect
...

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