docs: restructure element-based testing documentation

- Move API reference from element-based-testing.md to els.md
- Keep element-based-testing.md as a user guide with examples
- Update els.md import syntax to use ES6 imports
- Add complete WebElement API reference to els.md

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: DavertMik