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/src/content/docs/index.mdx

@@ -1,40 +1,50 @@
 ---
-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.
+title: RDBMS Playground
+description: A terminal playground for learning relational databases — tables, keys, relationships, indexes, queries, and query plans, hands-on.
+template: splash
 hero:
-  tagline: Congrats on setting up a new Starlight project!
-  image:
-    file: ../../assets/houston.webp
+  tagline: Learn relational databases by doing — in your terminal.
   actions:
-    - text: Example Guide
-      link: /guides/example/
+    - text: Get started
+      link: /getting-started/installation/
       icon: right-arrow
-    - text: Read the Starlight docs
-      link: https://starlight.astro.build
-      icon: external
+    - text: Browse the reference
+      link: /reference/types/
       variant: minimal
 ---
 
-import { Card, CardGrid } from '@astrojs/starlight/components';
+import { Card, CardGrid, LinkCard } from '@astrojs/starlight/components';
 
-## Next steps
+RDBMS Playground is a cross-platform terminal app that gives you a safe
+sandbox for exploring relational database concepts: tables, columns,
+primary and foreign keys, relationships, indexes, queries, and query
+plans. It is built for learning — from your first table to writing raw
+SQL.
 
-<CardGrid stagger>
-	<Card title="Update content" icon="pencil">
-		Edit `src/content/docs/index.mdx` to see this page change.
+<CardGrid>
+	<Card title="Two ways to work" icon="seti:db">
+		Start in **simple mode** — a friendly, keyword-based command language —
+		and switch to **advanced mode** for standard SQL whenever you are ready.
 	</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 title="Learn the SQL underneath" icon="open-book">
+		Run a simple-mode command in advanced mode and the playground shows you
+		the equivalent SQL, so the bridge from concepts to SQL is always visible.
 	</Card>
-	<Card title="Add new content" icon="add-document">
-		Add Markdown or MDX files to `src/content/docs` to create new pages.
+	<Card title="Safe to experiment" icon="approve-check">
+		Every change can be undone, your work is saved as you go, and you can
+		rebuild the whole database from plain, readable files.
 	</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 title="See how queries run" icon="rocket">
+		Ask the playground to explain any query and it renders the database's
+		plan as an annotated tree — so indexes and scans stop being a mystery.
 	</Card>
 </CardGrid>
+
+## Start here
+
+<CardGrid>
+	<LinkCard title="Install" href="/getting-started/installation/" description="Get the playground running on your machine." />
+	<LinkCard title="Your first project" href="/getting-started/first-project/" description="Create a table, add a row, and see it." />
+	<LinkCard title="The example library" href="/getting-started/example-library/" description="The small database used throughout these docs." />
+	<LinkCard title="Reference" href="/reference/types/" description="Every command, type, and constraint in detail." />
+</CardGrid>