Bump @antora/site-generator in /antora

Bumps [@antora/site-generator](https://gitlab.com/antora/antora) from 3.2.0-alpha.4 to 3.2.0-alpha.9.
- [Changelog](https://gitlab.com/antora/antora/blob/main/CHANGELOG.adoc)
- [Commits](https://gitlab.com/antora/antora/compare/v3.2.0-alpha.4...v3.2.0-alpha.9)

---
updated-dependencies:
- dependency-name: "@antora/site-generator"
  dependency-version: 3.2.0-alpha.9
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

Author: dependabot[bot]