docs(z-image) add Z-Image requirements and starter bundle

[lstein]

Summary

This PR updates the getting started and requirements docs to cover Z-Image. It also adds a Model Manager starter bundle for Z-Image. The bundle includes:

• Z-Image Turbo (quantized)
• Z-Image Qwen3 Encoder (quantized)
• Z-Image Controlnet Tile
• Z-Image Controlnet Union

Related Issues / Discussions

Just polishing up the release.

QA Instructions

Docs

1. Run "make docs", and view the docs on localhost:8080
2. Check the "Zimage" sections of the quick-start and installation pages

Starter Model Bundle

4. Go the Model Manager Models->Starter Models
5. Confirm that the Z-Image starter bundle is defined
6. Confirm that the listed bundled files install

Merge Plan

Should merge without difficulty

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Tests added / updated (if applicable)
• ❗Changes to a redux slice have a corresponding migration
• Documentation added / updated (if applicable)
• Updated What's New copy (if doing a release after this PR)

[Pfannkuchensack]

The zimage_bundle is missing the flux vae. There were some users who didn't quite understand that. Perhaps we should add that Flux VAE is also compatible/needed with z-image

[JPPhoto]

Please address the potential documentation errors.

[Pfannkuchensack]

Works fine :)