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/ui-tour.md

@@ -0,0 +1,29 @@
+# A tour of the window
+
+![The SolveCraft window](../generated/sample-plate.png)
+
+- **Title bar.** On Windows and Linux the application bar is the window's title bar: File, save,
+  undo and redo, the start page, the theme, Help, and one tab per open design (each with its own
+  undo history, selection and camera).
+- **Toolbar.** The workspace switcher (DESIGN) sits on the left. The tabs (SOLID, SURFACE,
+  SHEET METAL, PLASTIC, UTILITIES, and SKETCH while you sketch) hold panels of commands. Click a
+  panel's name for all its commands.
+- **Browser** (left). The design as a tree: document settings and units, named views, the
+  origin, bodies, sketches, construction planes, canvases, joints and components. The eye shows or
+  hides an item, a component's radio button makes it the active component, and right-clicking any
+  row gives that item's menu. Rows drag into groups and other components.
+- **Viewport.** Click to select, Shift or Ctrl to add, drag for a window or crossing box. The
+  wheel zooms at the cursor, the middle button pans and Shift+middle orbits (the mouse preset in
+  Preferences switches to SolidWorks or Inventor style). The view cube and the navigation bar
+  change the view. Right-click for the marking menu: eight commands around the cursor, plus a
+  menu for whatever is under it.
+- **Dialogs** dock on the right with a live preview. Most features also show drag handles and a
+  value box in the viewport.
+- **Timeline** (bottom). Every feature in order. Double-click to edit, drag the marker to roll
+  back, and drag features to reorder them.
+- **S box.** Press S anywhere for a command search at the cursor. Pin the commands you use most
+  to its top.
+
+Preferences (File › Preferences) hold the theme, the units of new designs, the mouse preset, the
+visual style and autosave. Help › Keyboard Shortcuts lists every key, and File › Keyboard
+Shortcuts changes them.