docs: initialize mdBook structure with stub pages and deployment workflow

- Add docs/book.toml with coal theme, repo link, and custom CSS
- Add docs/src/SUMMARY.md with navigation structure
- Create 16 content pages across guide/, architecture/, development/, reference/
- Add docs/theme/kindred.css with minimal sidebar overrides
- Add .gitea/workflows/docs.yml for auto-deploy on push to main
- Add docs/book/ to .gitignore

Pages are populated with real content from the codebase where possible.
Remaining pages have TODO markers for future content.

docs/src/development/contributing.md

@@ -0,0 +1,52 @@
+# Contributing
+
+Kindred Create is maintained at [git.kindred-systems.com/kindred/create](https://git.kindred-systems.com/kindred/create). Contributions are submitted as pull requests against the `main` branch.
+
+## Getting started
+
+```bash
+git clone --recursive ssh://git@git.kindred-systems.com:2222/kindred/create.git
+cd create
+pixi run configure
+pixi run build
+pixi run freecad
+```
+
+See [Building from Source](../guide/building.md) for the full development setup.
+
+## Branch and PR workflow
+
+1. Create a feature branch from `main`:
+   ```bash
+   git checkout -b feat/my-feature main
+   ```
+2. Make your changes, commit with conventional commit messages (see below).
+3. Push and open a pull request against `main`.
+4. CI builds and tests run automatically on all PRs.
+
+## Commit messages
+
+Use [Conventional Commits](https://www.conventionalcommits.org/):
+
+| Prefix | Purpose |
+|--------|---------|
+| `feat:` | New feature |
+| `fix:` | Bug fix |
+| `chore:` | Maintenance, dependencies |
+| `docs:` | Documentation only |
+| `art:` | Icons, theme, visual assets |
+
+Scope is optional but encouraged:
+- `feat(ztools): add datum point creation mode`
+- `fix(gui): correct menu icon size on Wayland`
+- `chore: update silo submodule`
+
+## Reporting issues
+
+Report issues at the [issue tracker](https://git.kindred-systems.com/kindred/create/issues). When reporting:
+
+1. Note whether the issue involves Kindred Create additions (ztools, Silo, theme) or base FreeCAD
+2. Include version info from **Help > About FreeCAD > Copy to clipboard**
+3. Provide reproduction steps and attach example files (FCStd as ZIP) if applicable
+
+For base FreeCAD issues, also check the [FreeCAD issue tracker](https://github.com/FreeCAD/FreeCAD/issues).