docs: Add end-to-end image pipeline example and regression test

[huleilei]

Changes Made

This PR improves the image modality documentation by adding a complete, copy-pasteable end-to-end pipeline example.

Documentation (docs/modalities/images.md):
- Added "End-to-End Image Pipeline" section covering: URL -> download -> decode -> resize -> to_tensor -> normalize (UDF).
- Added "UDF Best Practices for Images" section (handling None, return types, performance).

Related Issues

[greptile-apps]

Greptile Overview

Greptile Summary

Adds comprehensive documentation for image processing pipelines, including an end-to-end example covering URL download, decoding, resizing, tensor conversion, and normalization. Introduces UDF best practices section explaining None handling, return_dtype selection, performance considerations for NumPy/PyTorch vs PIL.Image, and batch processing techniques.

Confidence Score: 5/5

• Documentation-only changes with well-structured, accurate examples; safe to merge
• This PR only adds documentation with code examples. All API calls (download, decode_image, resize, image_to_tensor, @daft.func, @daft.func.batch) were verified against the codebase and are correct. The examples follow established patterns, handle edge cases (None values), and provide practical guidance for users. No bugs, logic errors, or security concerns identified.
• No files require special attention

Important Files Changed

File Analysis

Filename	Score	Overview
docs/modalities/images.md	5/5	Adds comprehensive end-to-end image pipeline example and UDF best practices documentation

Sequence Diagram

sequenceDiagram
    participant User
    participant Daft
    participant URL as Image URLs
    participant Tensor as Tensor Data
    
    User->>Daft: "Create DataFrame with URLs"
    Daft->>URL: "download(on_error='null')"
    URL-->>Daft: "Binary image data"
    Daft->>Daft: "decode_image()"
    Daft->>Daft: "resize(224, 224)"
    Daft->>Tensor: "image_to_tensor()"
    Tensor-->>Daft: "Tensor representation"
    Daft->>Daft: "normalize_image() UDF"
    Daft-->>User: "Normalized tensors ready for ML"

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[greptile-apps]

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[huleilei]

@universalmind303 @kevinzwang @colin-ho help me review when you are convenient. Thanks