build(deps-dev): bump vitepress in /docs

Bumps [vitepress](https://github.com/vuejs/vitepress) from 1.0.0-alpha.50 to 1.0.0-alpha.51.
- [Release notes](https://github.com/vuejs/vitepress/releases)
- [Changelog](https://github.com/vuejs/vitepress/blob/main/CHANGELOG.md)
- [Commits](vuejs/vitepress@v1.0.0-alpha.50...v1.0.0-alpha.51)

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

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

Author: dependabot[bot]