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/guides/build-the-library.md

@@ -0,0 +1,69 @@
+---
+title: Build the library
+description: Build the example library database step by step — tables, a relationship, and some rows.
+sidebar:
+  order: 1
+---
+
+:::note[Draft]
+This guide is an early draft. The walkthrough is correct, but the wording
+and pacing will be refined for teaching before the docs are published.
+:::
+
+This guide builds the [example library](/getting-started/example-library/)
+from scratch in simple mode: two tables, a relationship between them, and a
+few rows. By the end you will have used the create → add column → relate →
+insert → query loop end to end.
+
+## 1. Create the authors table
+
+```text
+create table authors with pk author_id(serial)
+add column to authors: name (text)
+add column to authors: birth_year (int)
+```
+
+## 2. Create the books table
+
+```text
+create table books with pk book_id(serial)
+add column to books: title (text)
+add column to books: author_id (int)
+add column to books: published (int)
+```
+
+## 3. Relate books to authors
+
+An author has many books, so `books.author_id` should point at
+`authors.author_id`. Declare that one-to-many relationship:
+
+```text
+add 1:n relationship from authors.author_id to books.author_id
+```
+
+The relationship reads parent-to-child: **from** the `authors` side **to**
+the `books` side.
+
+## 4. Add some rows
+
+`author_id` and `book_id` are `serial`, so they fill themselves in:
+
+```text
+insert into authors (name, birth_year) values ('Ada Lovelace', 1815)
+insert into authors (name, birth_year) values ('Alan Turing', 1912)
+insert into books (title, author_id, published) values ('Notes on the Analytical Engine', 1, 1843)
+```
+
+## 5. Look at your data
+
+```text
+show data authors
+show data books
+```
+
+## Where to go next
+
+- Add the `members` and `loans` tables the same way to model borrowing — a
+  many-to-many relationship through `loans`.
+- Try the same steps in advanced mode to see the SQL form of each command.
+- Look up any command in detail in the Reference section.