docs: add fox pet README and preview

[TamerMansour-AI]

Summary

• add an original README/how-to guide for the fox pet package with package contents, hosted/local scene usage, pet metadata, replacement steps, testing checklist, and troubleshooting
• add a standalone model-viewer preview at examples/fox-scene.html
• add docs/fox-preview.png generated from the loaded fox_uastc.glb preview

Validation

• parsed .metaversefile as JSON
• verified fox_uastc.glb and fox.glb headers are glTF v2 and declared lengths match file lengths
• ran git diff --check
• served the repo locally with python -m http.server 8185
• opened examples/fox-scene.html and confirmed model-viewer loaded ../fox_uastc.glb successfully
• generated the README preview image from the loaded model-viewer scene

Notes: the browser preview validates the optimized GLB asset path and visual. Full pet follow/look/animation behavior still needs a Webaverse-compatible runtime, which is documented in the README checklist.