Fix custom components documentation to match actual API

[Aaronontheweb]

Summary

• Fixed ILayoutNode interface to use correct property names (WidthConstraint/HeightConstraint instead of Width/Height)
• Fixed Render method signature to use Rect instead of ScreenBounds
• Fixed bounds properties to use X/Y instead of Left/Top
• Fixed render methods to use WriteAt() with SetForeground()/SetBackground() instead of non-existent SetCell()
• Added new "Accessing Dimensions" section explaining how to get width/height from Rect bounds, IRenderContext, and Size available
• Updated all code examples to properly extend LayoutNode base class
• Documented fluent size methods available on LayoutNode base class
• Updated best practices to reference correct IRenderContext methods

Test plan

• Review documentation renders correctly
• Verify code examples compile against actual API