chore(deps): bump @astrojs/starlight from 0.17.4 to 0.21.3 in /site

Bumps [@astrojs/starlight](https://github.com/withastro/starlight/tree/HEAD/packages/starlight) from 0.17.4 to 0.21.3.
- [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@0.21.3/packages/starlight)

---
updated-dependencies:
- dependency-name: "@astrojs/starlight"
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]

site/package.json

@@ -10,7 +10,7 @@
     "astro": "astro"
   },
   "dependencies": {
-    "@astrojs/starlight": "^0.17.4",
+    "@astrojs/starlight": "^0.21.3",
     "astro": "^4.5.12",
     "sharp": "^0.33.2",
     "@astrojs/check": "^0.5.6",