build(deps): bump cross-spawn, nextra and nextra-theme-docs in /docs

Bumps [cross-spawn](https://github.com/moxystudio/node-cross-spawn) to 7.0.6 and updates ancestor dependencies [cross-spawn](https://github.com/moxystudio/node-cross-spawn), [nextra](https://github.com/shuding/nextra) and [nextra-theme-docs](https://github.com/shuding/nextra). These dependencies need to be updated together.


Updates `cross-spawn` from 7.0.3 to 7.0.6
- [Changelog](https://github.com/moxystudio/node-cross-spawn/blob/master/CHANGELOG.md)
- [Commits](moxystudio/node-cross-spawn@v7.0.3...v7.0.6)

Updates `nextra` from 2.13.2 to 4.2.13
- [Release notes](https://github.com/shuding/nextra/releases)
- [Commits](https://github.com/shuding/nextra/compare/[email protected]@4.2.13)

Updates `nextra-theme-docs` from 2.13.2 to 4.2.13
- [Release notes](https://github.com/shuding/nextra/releases)
- [Commits](https://github.com/shuding/nextra/compare/[email protected]@4.2.13)

---
updated-dependencies:
- dependency-name: cross-spawn
  dependency-type: indirect
- dependency-name: nextra
  dependency-type: direct:production
- dependency-name: nextra-theme-docs
  dependency-type: direct:production
...

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

Author: dependabot[bot]