Documentation site and CI workflows

An mdBook under docs/book: a user guide (getting started, UI tour, sketching, features,
parameters, components and joints, sheet metal, plastic, files, shortcuts, MCP) and a developer
guide (CONTRIBUTING, architecture, command engine, kernel boundary, persistent naming, test
harnesses, and the existing docs/*.md pages). `cargo xtask book` generates the command reference
from `solvecraft-cli commands` and renders the screenshots with `solvecraft-cli snapshot`, then
runs mdbook. .github/workflows/pages.yml publishes the book at /solvecraft/ and the web app at
/solvecraft/app/ (skipped while the repository is private). .github/workflows/ci.yml runs
`cargo xtask ci` on pull requests. README and Help → Documentation link to the book.

Fixes #28
Fixes #8

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/book/src/guide/getting-started.md

@@ -0,0 +1,56 @@
+# Getting started
+
+## Desktop
+
+Download the latest build from the
+[releases page](https://github.com/bherbruck/solvecraft/releases), or build it yourself with
+stable Rust (1.90 or newer):
+
+```sh
+git clone https://github.com/bherbruck/solvecraft && cd solvecraft
+cargo run --release -p solvecraft -- --sample   # opens with a sample part
+```
+
+`solvecraft part.step` opens a STEP, IGES, 3MF, STL or OBJ file as a new design, and
+`solvecraft design.solvecraft` opens a saved design. Windows builds cross-compile from Linux with
+`cargo xwin build --release --target x86_64-pc-windows-msvc -p solvecraft`.
+
+## Web
+
+The same app runs in the browser: [open SolveCraft on the web](../app/) (`?sample` opens the sample
+part, `?webgl` forces WebGL2 where WebGPU is missing). Designs are saved in the browser's private
+file storage, which needs HTTPS or localhost. To build it yourself:
+
+```sh
+rustup target add wasm32-unknown-unknown
+cd apps/solvecraft-web && trunk serve --release
+```
+
+## CLI
+
+`solvecraft-cli` runs the same commands without a window:
+
+```sh
+cargo run --release -p solvecraft-cli -- commands                       # every command, as JSON
+cargo run --release -p solvecraft-cli -- run examples/bracket.json --out bracket.step
+cargo run --release -p solvecraft-cli -- eval examples/bracket.json     # volume, area, faces…
+cargo run --release -p solvecraft-cli -- snapshot examples/bracket.json --out bracket.png
+cargo run --release -p solvecraft-cli -- exec solid.box '{"length": 20, "width": 10, "height": 5}'
+```
+
+A script is a list of commands:
+
+```json
+{"commands": [
+  {"command": "sketch.create", "params": {"plane": "XY"}},
+  {"command": "sketch.rectangle.two_point", "params": {"p0": [0, 0], "p1": [40, 30]}},
+  {"command": "sketch.finish", "params": {}},
+  {"command": "solid.extrude", "params": {"distance": 20}},
+  {"command": "solid.fillet", "params": {"edges": [[0, 0, 10]], "radius": 3}}
+]}
+```
+
+![The bracket example, rendered by solvecraft-cli snapshot](../generated/bracket.png)
+
+Units are millimetres and degrees unless an expression says otherwise (`"1 in"`, `"30 deg"`),
+and Z is up.