Add comprehensive GitHub Actions CI/CD documentation

Add new documentation page covering GitHub Actions integration with psake:
- Complete workflow examples with cross-platform support
- Multiple psake installation methods including caching strategies
- Secret management best practices and examples
- Matrix builds across Windows, Linux, macOS and PowerShell versions
- Artifact publishing patterns for NuGet, GitHub Releases
- Example repository structure and troubleshooting guide
- Advanced patterns including reusable workflows

Update sidebars.ts to include the new page in CI Examples category.

This addresses Phase 1, Task 1.1 of the documentation expansion plan.

Author: claude