Add Copy Page dropdown feature with AI assistant integration

- Create CopyPageDropdown component with three options:
  - Copy page as Markdown for LLMs
  - Open in Claude with pre-filled prompt
  - Open in ChatGPT with pre-filled prompt
- Add API endpoint to fetch and clean page content (getPageContent.ts)
- Integrate dropdown into all English documentation pages via theme.config.tsx
- Use official Claude AI and ChatGPT logo SVGs
- Position dropdown in top-right corner of content area
- Handle path construction for different page routes (.en suffix detection)
- Add error handling and logging for troubleshooting

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>