docs: Create Responsive Design and Dark Mode Example Page

## Description
Create an example page showcasing responsive breakpoints and dark mode support.

## Features to Showcase
- [ ] Breakpoint modifiers: sm:, md:, lg:, xl:, 2xl:
- [ ] Responsive layout changes
- [ ] Platform modifiers: ios:, android:, web:, mobile:
- [ ] Dark mode: dark: modifier
- [ ] Dark mode toggle implementation
- [ ] WindThemeData brightness configuration