Bump js-yaml and postcss-cli in /docs

Removes [js-yaml](https://github.com/nodeca/js-yaml). It's no longer used after updating ancestor dependency [postcss-cli](https://github.com/postcss/postcss-cli). These dependencies need to be updated together.


Removes `js-yaml`

Updates `postcss-cli` from 7.1.1 to 11.0.1
- [Release notes](https://github.com/postcss/postcss-cli/releases)
- [Changelog](https://github.com/postcss/postcss-cli/blob/master/CHANGELOG.md)
- [Commits](postcss/postcss-cli@7.1.1...11.0.1)

---
updated-dependencies:
- dependency-name: js-yaml
  dependency-version: 
  dependency-type: indirect
- dependency-name: postcss-cli
  dependency-version: 11.0.1
  dependency-type: direct:development
...

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

Author: dependabot[bot]