docs(deps)(deps): bump astro and @astrojs/starlight in /docs

Bumps [astro](https://github.com/withastro/astro/tree/HEAD/packages/astro) and [@astrojs/starlight](https://github.com/withastro/starlight/tree/HEAD/packages/starlight). These dependencies needed to be updated together.

Updates `astro` from 5.18.1 to 6.3.1
- [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@6.3.1/packages/astro)

Updates `@astrojs/starlight` from 0.30.6 to 0.39.2
- [Release notes](https://github.com/withastro/starlight/releases)
- [Changelog](https://github.com/withastro/starlight/blob/main/packages/starlight/CHANGELOG.md)
- [Commits](https://github.com/withastro/starlight/commits/@astrojs/starlight@0.39.2/packages/starlight)

---
updated-dependencies:
- dependency-name: astro
  dependency-version: 6.3.1
  dependency-type: direct:production
- dependency-name: "@astrojs/starlight"
  dependency-version: 0.39.2
  dependency-type: direct:production
...

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Author: dependabot[bot]