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/src/content/docs/index.mdx

@@ -0,0 +1,40 @@
+---
+title: Welcome to Starlight
+description: Get started building your docs site with Starlight.
+template: splash # Remove or comment out this line to display the site sidebar on this page.
+hero:
+  tagline: Congrats on setting up a new Starlight project!
+  image:
+    file: ../../assets/houston.webp
+  actions:
+    - text: Example Guide
+      link: /guides/example/
+      icon: right-arrow
+    - text: Read the Starlight docs
+      link: https://starlight.astro.build
+      icon: external
+      variant: minimal
+---
+
+import { Card, CardGrid } from '@astrojs/starlight/components';
+
+## Next steps
+
+<CardGrid stagger>
+	<Card title="Update content" icon="pencil">
+		Edit `src/content/docs/index.mdx` to see this page change.
+	</Card>
+	<Card title="Change page layout" icon="document">
+		Delete `template: splash` in `src/content/docs/index.mdx` to display a
+		sidebar on this page.
+	</Card>
+	<Card title="Add new content" icon="add-document">
+		Add Markdown or MDX files to `src/content/docs` to create new pages.
+	</Card>
+	<Card title="Configure your site" icon="setting">
+		Edit your `sidebar` and other config in `astro.config.mjs`.
+	</Card>
+	<Card title="Read the docs" icon="open-book">
+		Learn more in [the Starlight Docs](https://starlight.astro.build/).
+	</Card>
+</CardGrid>