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/installation.md

@@ -0,0 +1,64 @@
+---
+title: Installation
+description: Install RDBMS Playground from a prebuilt binary or a package manager, and run it for the first time.
+sidebar:
+  order: 1
+---
+
+RDBMS Playground is a single self-contained program. There is nothing to
+configure and no separate database to install — everything it needs is
+built in.
+
+## Prebuilt binaries
+
+Download the binary for your platform, make it executable if needed, and
+put it somewhere on your `PATH`.
+
+:::note
+Download links are published with each release. They are added here when the
+first public version ships.
+:::
+
+## Package managers
+
+Once published, the playground will be installable through common package
+managers:
+
+```sh
+# macOS / Linux (Homebrew)
+brew install rdbms-playground
+
+# Windows (Scoop)
+scoop install rdbms-playground
+```
+
+:::note
+Package-manager availability lands with the first public release; the exact
+names are confirmed here at that time.
+:::
+
+## Run it
+
+Start the playground with no arguments and it opens a fresh, automatically
+named temporary project so you can start experimenting immediately:
+
+```sh
+rdbms-playground
+```
+
+To open an existing project, pass its path:
+
+```sh
+rdbms-playground path/to/project
+```
+
+Useful options (run `rdbms-playground --help` for the full list):
+
+| Option | What it does |
+|---|---|
+| `--resume` | Reopen the most recently used project. |
+| `--data-dir <PATH>` | Use a different location for stored projects. |
+| `--theme <light\|dark>` | Force a theme instead of auto-detecting. |
+| `--mode <simple\|advanced>` | Start in a specific input mode. |
+
+Next: [create your first project](/getting-started/first-project/).