Bump postcss and autoprefixer in /docs

Bumps [postcss](https://github.com/postcss/postcss) to 8.5.6 and updates ancestor dependency [autoprefixer](https://github.com/postcss/autoprefixer). These dependencies need to be updated together.


Updates `postcss` from 7.0.39 to 8.5.6
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](postcss/postcss@7.0.39...8.5.6)

Updates `autoprefixer` from 9.8.0 to 10.4.23
- [Release notes](https://github.com/postcss/autoprefixer/releases)
- [Changelog](https://github.com/postcss/autoprefixer/blob/main/CHANGELOG.md)
- [Commits](postcss/autoprefixer@9.8.0...10.4.23)

---
updated-dependencies:
- dependency-name: postcss
  dependency-version: 8.5.6
  dependency-type: indirect
- dependency-name: autoprefixer
  dependency-version: 10.4.23
  dependency-type: direct:development
...

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

Author: dependabot[bot]