feat: Add comprehensive DocC documentation with tutorials

Add complete Apple-style DocC documentation for TUIKit framework:

## Documentation Structure
- Main landing page (TUIKit.md) with quick-start and topic navigation
- 8 guide articles: GettingStarted, ViewHierarchy, StateManagement, Theming, Appearance, Focus, Modifiers, Architecture
- 3 step-by-step tutorials: BuildYourFirstApp, BuildInteractiveMenu, BuildThemableUI
- Documentation catalog with proper Info.plist configuration

## Code Improvements
- Enhanced Box.swift documentation with clear container comparison
- Added comprehensive examples and usage patterns
- Explained appearance integration and sizing behavior

## Infrastructure
- GitHub Actions workflow (.github/workflows/docc.yml) for automatic DocC building
- Configured for deployment to GitHub Pages on push to main
- Uses xcrun docc build with proper artifact handling

## Documentation Includes
- API reference stubs for all major components
- Comprehensive guide articles with code examples
- Interactive tutorials following Apple's documentation style
- Cross-references using DocC syntax (``Type``)
- Best practices and design patterns explained

The documentation is ready for automated building and deployment to GitHub Pages.

.github/workflows/docc.yml

@@ -0,0 +1,47 @@
+name: Build and Deploy DocC Documentation
+
+on:
+  push:
+    branches:
+      - main
+  pull_request:
+    branches:
+      - main
+  workflow_dispatch:
+
+permissions:
+  contents: read
+  pages: write
+  id-token: write
+
+concurrency:
+  group: "pages"
+  cancel-in-progress: false
+
+jobs:
+  build-and-deploy:
+    runs-on: macos-latest
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+
+      - name: Set up Swift
+        uses: swift-actions/setup-swift@v1
+        with:
+          swift-version: '6.0'
+
+      - name: Build documentation
+        run: |
+          xcrun docc build \
+            --product TUIKit \
+            --output-path ./docs
+
+      - name: Upload Pages artifact
+        uses: actions/upload-pages-artifact@v3
+        with:
+          path: './docs'
+
+      - name: Deploy to GitHub Pages
+        id: deployment
+        uses: actions/deploy-pages@v4
+        if: github.event_name == 'push' && github.ref == 'refs/heads/main'