[docs]: Bump astro from 4.15.1 to 4.15.9 in /docs

Bumps [astro](https://github.com/withastro/astro/tree/HEAD/packages/astro) from 4.15.1 to 4.15.9.
- [Release notes](https://github.com/withastro/astro/releases)
- [Changelog](https://github.com/withastro/astro/blob/main/packages/astro/CHANGELOG.md)
- [Commits](https://github.com/withastro/astro/commits/[email protected]/packages/astro)

---
updated-dependencies:
- dependency-name: astro
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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