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/getting-started/example-library.md

@@ -0,0 +1,32 @@
+---
+title: The example library
+description: The small library database used throughout these docs.
+sidebar:
+  order: 4
+---
+
+Most examples in this documentation use the same small database: a library
+with **authors**, **books**, **members**, and **loans**. Keeping one
+familiar set of tables makes it easier to focus on the concept each page is
+teaching.
+
+## The tables
+
+| Table | Columns |
+|---|---|
+| `authors` | `author_id` (serial, primary key), `name` (text), `birth_year` (int) |
+| `books` | `book_id` (serial, primary key), `title` (text), `author_id` (int → authors), `published` (int), `isbn` (text, unique) |
+| `members` | `member_id` (serial, primary key), `name` (text), `joined` (date) |
+| `loans` | `loan_id` (serial, primary key), `book_id` (int → books), `member_id` (int → members), `loaned_on` (date), `returned_on` (date) |
+
+## The relationships
+
+- **An author has many books.** `books.author_id` points at
+  `authors.author_id` — a one-to-many relationship.
+- **Members borrow books.** Each row in `loans` ties one book to one member,
+  so `loans` connects `books` and `members` — a many-to-many relationship
+  expressed through a bridging table.
+
+You will build these tables and relationships step by step in
+[Build the library](/guides/build-the-library/). Individual reference pages
+use whichever part of this schema illustrates the command at hand.