docs: website docs structure + first content pages

Phase D foundation. Configures the pragmatic four-section sidebar
(Getting started / Guides / Reference / Concepts) and replaces the
template example pages with grounded content built on the shared
"library" example database (authors/books/members/loans):

- Getting started: installation, first project, simple vs advanced,
  the example library.
- Reference: Types (all ten + serial/shortid + advanced aliases),
  Tables (create/drop, compound PK, advanced CREATE TABLE).
- Concepts: projects & storage (readable files, derived database,
  autosave, temp projects).
- Guides: Build the library (draft, to be refined for teaching).

Command syntax grounded in en-US.yaml usage/help, command.rs, and
types.rs (verified against tests). Records the settled doc decisions
in STYLE.md. Build green (10 pages, Pagefind); content clean of
"DSL"/engine-name.

website/astro.config.mjs

@@ -6,28 +6,30 @@ 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' } }],
-              },
-          ],
-      }),
+	// TODO(Phase B/SEO): set `site` to the production URL once the domain is
+	// known — enables the sitemap and canonical/OG URLs.
+	integrations: [
+		starlight({
+			title: 'RDBMS Playground',
+			tagline: 'Learn relational databases by doing.',
+			// TODO(Phase B): point at the real repository once it moves to its
+			// public home. Omitted for now rather than linking the wrong repo.
+			// social: [{ icon: 'github', label: 'GitHub', href: '…' }],
+			customCss: ['./src/styles/global.css'],
+			// Pragmatic structure (ADR-0042 §7 / website/STYLE.md): Getting
+			// started, Guides, Reference, Concepts. Autogenerated per directory;
+			// in-section order is controlled by each page's `sidebar.order`
+			// frontmatter.
+			sidebar: [
+				{ label: 'Getting started', items: [{ autogenerate: { directory: 'getting-started' } }] },
+				{ label: 'Guides', items: [{ autogenerate: { directory: 'guides' } }] },
+				{ label: 'Reference', items: [{ autogenerate: { directory: 'reference' } }] },
+				{ label: 'Concepts', items: [{ autogenerate: { directory: 'concepts' } }] },
+			],
+		}),
 	],
 
-  vite: {
-    plugins: [tailwindcss()],
-  },
-});
+	vite: {
+		plugins: [tailwindcss()],
+	},
+});