Upgrade Astro and dependencies, update GitHub page (#1824)

* Upgrade Astro and dependencies, update GitHub page

Bump Astro and related packages and update the site to match. package.json and package-lock.json updated (astro -> 6.0.4, @astrojs/starlight -> 0.38.1, @astrojs/mdx/markdown-remark/prism/sitemap and other plugins bumped) and Node engine minimum raised to >=22.12.0. The lockfile also contains many transitive dependency updates (esbuild, babel, @expressive-code, etc.). Minor changes to website/src/pages/github/index.astro to align with the dependency upgrades and updated plugin behavior.

* Update website.yml

Author: coliff

.github/workflows/website.yml

@@ -33,6 +33,7 @@ jobs:
       - name: ⎔ Setup node
         uses: actions/setup-node@6044e13b5dc448c55e2357c09f80417699197238 # v6.2.0
         with:
+          node-version: '22'
           cache: npm
           cache-dependency-path: website/package-lock.json