Bump postcss and gatsby in /docs

Bumps [postcss](https://github.com/postcss/postcss) to 8.4.31 and updates ancestor dependency [gatsby](https://github.com/gatsbyjs/gatsby). These dependencies need to be updated together.


Updates `postcss` from 7.0.27 to 8.4.31
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](postcss/postcss@7.0.27...8.4.31)

Updates `gatsby` from 2.21.33 to 5.12.5
- [Release notes](https://github.com/gatsbyjs/gatsby/releases)
- [Changelog](https://github.com/gatsbyjs/gatsby/blob/master/CHANGELOG.md)
- [Commits](https://github.com/gatsbyjs/gatsby/compare/[email protected]@5.12.5)

---
updated-dependencies:
- dependency-name: postcss
  dependency-type: indirect
- dependency-name: gatsby
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>