Bump postcss, next and nextra-theme-docs

Bumps [postcss](https://github.com/postcss/postcss) to 8.4.31 and updates ancestor dependencies [postcss](https://github.com/postcss/postcss), [next](https://github.com/vercel/next.js) and [nextra-theme-docs](https://github.com/shuding/nextra). These dependencies need to be updated together.


Updates `postcss` from 8.4.5 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@8.4.5...8.4.31)

Updates `next` from 12.1.0 to 13.5.4
- [Release notes](https://github.com/vercel/next.js/releases)
- [Changelog](https://github.com/vercel/next.js/blob/canary/release.js)
- [Commits](vercel/next.js@v12.1.0...v13.5.4)

Updates `nextra-theme-docs` from 1.2.6 to 2.13.2
- [Release notes](https://github.com/shuding/nextra/releases)
- [Commits](https://github.com/shuding/nextra/commits/nextra-theme-docs@2.13.2)

---
updated-dependencies:
- dependency-name: postcss
  dependency-type: indirect
- dependency-name: next
  dependency-type: direct:production
- dependency-name: nextra-theme-docs
  dependency-type: direct:production
...

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