feat: add portable windows build script and documentation

[Bluear7878]

Motivation

Provide an easy way for Windows users to build their own portable ComfyUI-nunchaku environment.

Modifications

• Add portable_windows/build_portable.cmd: Interactive build script for creating portable
  Windows builds
  • Add portable_windows/templates/run_nvidia_gpu.bat: Launcher script for the portable build
  • Update docs/source/get_started/installation.rst: Add Option 4 documenting the portable windows build
    process

Checklist

• Code is formatted using Pre-Commit hooks (run pre-commit run --all-files).
• Relevant unit tests are added in the tests/workflows directory following the guidance in the Contribution Guide.
• Reference images are uploaded to PR comments and URLs are added to test_cases.json.
• Additional test data (if needed) is registered in test_data/inputs.yaml.
• Additional models (if needed) are registered in scripts/download_models.py and test_data/models.yaml.
• Additional custom nodes (if needed) are added to .github/workflows/pr-test.yaml.
• For reviewers: If you're only helping merge the main branch and haven't contributed code to this PR, please remove yourself as a co-author when merging.
• Please feel free to join our Discord or WeChat to discuss your PR.

[lmxyy]

We should not have users build the portable themselves. Instead, we should add the portable build to our CI pipeline and ship pre-built artifacts.