build(deps-dev): bump autoprefixer from 10.2.6 to 10.4.0 in /docs

Bumps [autoprefixer](https://github.com/postcss/autoprefixer) from 10.2.6 to 10.4.0.
- [Release notes](https://github.com/postcss/autoprefixer/releases)
- [Changelog](https://github.com/postcss/autoprefixer/blob/main/CHANGELOG.md)
- [Commits](postcss/autoprefixer@10.2.6...10.4.0)

---
updated-dependencies:
- dependency-name: autoprefixer
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]