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/assemblies.md

@@ -0,0 +1,22 @@
+# Components and joints
+
+A design is a tree of **components**. **New Component** (`component.create`) adds one, and
+**Create Components from Bodies** (`component.from_bodies`) turns bodies into components. Each
+component has its own origin, bodies, sketches and timeline entries. The **active component**
+(the radio button in the browser) receives new sketches and features; the others are drawn
+faded.
+
+A component is placed by its **occurrences**. Paste creates another instance of the same
+component, and **Paste New** creates an independent copy. Move an occurrence with the move
+triad, then **Capture Position** to keep it or **Revert** to put it back. SolveCraft asks before
+the next command if a move wasn't captured. **Ground** fixes an occurrence in place.
+
+**Joints** (`joint.create`, J) connect two components by joint origins that snap to faces, edges,
+circles and vertices: rigid, revolute, slider, cylindrical, pin-slot, planar and ball, with
+offsets, flip and limits. **As-built Joint** keeps the current position, **Rigid Group** locks
+components together, **Drive Joints** moves a joint through its values, and **Motion Link**
+couples two joints. Contact sets stop motion where bodies would pass through each other,
+**Motion Study** keys joint values along a timeline, and **Interference** shows overlapping
+volumes in red.
+
+STEP export writes the component tree as an assembly.