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/parameters.md

@@ -0,0 +1,21 @@
+# Parameters and expressions
+
+Every dimension and feature input is a **model parameter** (`d1`, `d2`, …). **User parameters**
+are ones you add yourself, with a unit, an expression and an optional comment. Change them in
+**Change Parameters** (`parameters.change`). The timeline rebuilds from the first feature that
+uses them.
+
+Expressions are unit-aware: `2 * width + 5 mm`, `angle / 2`, `sqrt(area)`, `1 in + 3 mm`, using
+`+ - * / ^`, parentheses, `sin cos tan asin acos atan atan2 sqrt abs min max floor ceil round mod
+trunc` and unit conversion. A length where an angle is expected (or the other way round) is an
+error, not a silent conversion. Cycles are reported with their path (`a → b → a`), renaming a
+parameter updates every expression, dimension and feature input that uses it, and a parameter
+in use can't be deleted (the error lists its users).
+
+The design's units (Document Settings in the browser: mm, cm, m, in or ft) set how a bare number
+typed into a feature is read. In an inch design `2` means 2 in. Changing the units never moves
+existing geometry.
+
+Parameters export to and import from CSV or JSON (`parameters.export`, `parameters.import`).
+**Configurations** (`config.*`) keep a table of rows, each with its own parameter values and
+suppressed features, switched with `config.activate`.