docs: restructure plugin docs with multi-file layout and seed reader type

[johnnygreco]

📋 Summary

Restructures the plugin documentation to recommend a multi-file layout (config.py, impl.py, plugin.py) and documents seed reader plugins as a second supported plugin type.

🔄 Changes

🔧 Changed

• Plugin example (docs/plugins/example.md): Refactored from a single-file plugin layout to a multi-file structure with separate config.py, impl.py, and plugin.py files. Updated terminology ("Task Class" → "Implementation Class"), expanded the config class example with get_column_emoji(), required_columns, and side_effect_columns, and removed the redundant "Complete plugin code" section.
• Plugin overview (docs/plugins/overview.md): Updated to list both column generator and seed reader plugin types, and revised the "Implement the Plugin Components" section to describe the recommended multi-file structure.

✨ Added

• Validating Your Plugin section — documents assert_valid_plugin testing utility
• Multiple Plugins in One Package section — explains how to register multiple plugins via entry points, with a link to tests_e2e/ as a reference

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[greptile-apps]

Greptile Overview

Greptile Summary

This PR restructures the plugin documentation to recommend a multi-file layout and adds seed reader plugins as a second supported plugin type.

Key Documentation Updates:

• Multi-file structure: Recommends separating plugins into config.py, impl.py, and plugin.py files for better organization, testability, and to avoid circular dependencies
• Terminology update: Changed "Task Class" to "Implementation Class" for clarity
• Expanded config examples: Added get_column_emoji(), required_columns, and side_effect_columns properties to config class examples
• Seed reader support: Documents seed reader plugins alongside column generator plugins, with references to SeedSource and SeedReader base classes
• New sections: Added "Validating Your Plugin" (using assert_valid_plugin) and "Multiple Plugins in One Package" sections

The documentation changes are accurate and align with the actual plugin implementation in tests_e2e/. The multi-file structure matches the test plugin layout, and all referenced classes (ColumnGeneratorFullColumn, SeedReader, SeedSource, etc.) exist in the codebase.

Confidence Score: 5/5

• This PR is safe to merge with no risks - it only updates documentation files
• The PR exclusively modifies documentation files (.md) with no code changes. All documentation updates accurately reflect the actual codebase implementation, verified against test plugins in tests_e2e/. The recommended multi-file structure, class names, and methods all match the real implementations.
• No files require special attention

Important Files Changed

Filename	Overview
docs/plugins/example.md	Restructured plugin documentation from single-file to multi-file layout (config.py, impl.py, plugin.py). Updated terminology, expanded config examples with get_column_emoji(), required_columns, side_effect_columns. Added sections for plugin validation and multiple plugins per package.
docs/plugins/overview.md	Updated to document both column generator and seed reader plugin types. Revised plugin component descriptions to recommend multi-file structure and updated base classes.

Sequence Diagram

sequenceDiagram
    participant Dev as Plugin Developer
    participant Doc as Documentation
    participant Pkg as Plugin Package
    participant DD as DataDesigner

    Note over Dev,DD: Plugin Development Workflow (New Multi-File Structure)
    
    Dev->>Doc: Read restructured plugin docs
    Doc-->>Dev: Shows multi-file layout recommendation
    
    Dev->>Pkg: Create config.py
    Note over Pkg: Defines SingleColumnConfig/SeedSource<br/>with discriminator field
    
    Dev->>Pkg: Create impl.py
    Note over Pkg: Implements ColumnGenerator or SeedReader
    
    Dev->>Pkg: Create plugin.py
    Note over Pkg: Instantiates Plugin object<br/>connecting config and impl
    
    Dev->>Pkg: Define pyproject.toml entry point
    Note over Pkg: Registers under data_designer.plugins
    
    Dev->>DD: Install plugin (uv pip install -e .)
    DD->>Pkg: Discover plugin via entry point
    Pkg-->>DD: Load Plugin instance
    
    Dev->>DD: Use plugin in config builder
    Note over DD: Plugin works like built-in types
    
    Dev->>DD: Validate with assert_valid_plugin()
    DD-->>Dev: Confirms correct structure

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