build(deps-dev): bump vitepress from 0.15.6 to 0.20.0 in /docs

Bumps [vitepress](https://github.com/vuejs/vitepress) from 0.15.6 to 0.20.0.
- [Release notes](https://github.com/vuejs/vitepress/releases)
- [Changelog](https://github.com/vuejs/vitepress/blob/main/CHANGELOG.md)
- [Commits](vuejs/vitepress@v0.15.6...v0.20.0)

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

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

Author: dependabot[bot]