docs(deps): bump vitepress in /docs

Bumps [vitepress](https://github.com/vuejs/vitepress) from 2.0.0-alpha.17 to 2.0.0-alpha.18.
- [Release notes](https://github.com/vuejs/vitepress/releases)
- [Changelog](https://github.com/vuejs/vitepress/blob/main/CHANGELOG.md)
- [Commits](vuejs/vitepress@v2.0.0-alpha.17...v2.0.0-alpha.18)

---
updated-dependencies:
- dependency-name: vitepress
  dependency-version: 2.0.0-alpha.18
  dependency-type: direct:development
  update-type: version-update:semver-patch
...

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

Author: dependabot[bot]