docs(waiter): Add comprehensive documentation for terminal content waiters

- Create detailed markdown documentation in docs/test-helpers/waiter.md
- Add key features section highlighting main capabilities
- Include quick start examples for all functions
- Document fluent API with Playwright-inspired design
- Explain wait_for_any_content and wait_for_all_content with practical examples
- Add detailed API reference for all waiters
- Include testing best practices section

Author: tony