chore: scaffold website (Astro 6 + Starlight + Tailwind v4)

Phase A of docs/plans/20260604-adr-0042-website.md. Scaffolds the site
under website/ from the Starlight template; adds Tailwind v4 (via
@tailwindcss/vite) bridged to Starlight with @astrojs/starlight-tailwind
(src/styles/global.css + customCss). Production build is green: static
output, Pagefind search index, sharp image optimization.

Template placeholders (title, example pages, sidebar) are left for
Phase B/D. Reconciles the ADR/plan/index wording from "Astro 5" to
"Astro 6" to match the scaffolded toolchain.

website/astro.config.mjs

@@ -0,0 +1,33 @@
+// @ts-check
+import { defineConfig } from 'astro/config';
+import starlight from '@astrojs/starlight';
+
+import tailwindcss from '@tailwindcss/vite';
+
+// https://astro.build/config
+export default defineConfig({
+  integrations: [
+      starlight({
+          title: 'My Docs',
+          social: [{ icon: 'github', label: 'GitHub', href: 'https://github.com/withastro/starlight' }],
+          customCss: ['./src/styles/global.css'],
+          sidebar: [
+              {
+                  label: 'Guides',
+                  items: [
+                      // Each item here is one entry in the navigation menu.
+                      { label: 'Example Guide', slug: 'guides/example' },
+                  ],
+              },
+              {
+                  label: 'Reference',
+                  items: [{ autogenerate: { directory: 'reference' } }],
+              },
+          ],
+      }),
+	],
+
+  vite: {
+    plugins: [tailwindcss()],
+  },
+});