Bump astro from 5.8.1 to 5.15.6 in /docs

Bumps [astro](https://github.com/withastro/astro/tree/HEAD/packages/astro) from 5.8.1 to 5.15.6.
- [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/astro@5.15.6/packages/astro)

---
updated-dependencies:
- dependency-name: astro
  dependency-version: 5.15.6
  dependency-type: direct:production
...

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Author: dependabot[bot]