Add comprehensive developer documentation for ProBuilder codebase

[Copilot]

This PR adds extensive developer documentation to help developers understand ProBuilder's architecture, algorithms, and development workflows. The documentation is organized in a new Developer-Docs/ folder with multiple focused documents.

What's Added

The documentation covers ProBuilder's complete architecture from a developer perspective:

• System Architecture: High-level overview of the modular design with Runtime, Editor, and External library layers
• Core Components: Deep dive into fundamental classes like ProBuilderMesh, Face, Edge, Vertex, and the sophisticated shared vertex system
• Mesh Operations: Detailed coverage of algorithms including triangulation, extrusion, subdivision, and CSG operations
• Editor Tools: Complete guide to the editor architecture, selection system, and Unity integration
• Development Guide: Practical workflows for bug fixes, feature development, and performance optimization

Key Algorithm Documentation

The documentation includes detailed explanations and external references for the major algorithms used:

• Delaunay Triangulation: ProBuilder uses the Poly2Tri library implementing sweep line triangulation
• Constructive Solid Geometry: BSP tree-based boolean operations for mesh combination
• Winged Edge Data Structure: Advanced topology navigation system for efficient mesh traversal
• k-d Trees: Spatial indexing for performance-critical operations like vertex welding

Each algorithm section includes links to authoritative sources and explains any differences from standard implementations.

Structure

Developer-Docs/
├── README.md              # Navigation and quick start
├── Overview.md            # System architecture overview  
├── Core-Components.md     # Fundamental classes and data structures
├── Mesh-Operations.md     # Algorithms and mesh manipulation
├── Editor-Tools.md        # Editor architecture and tools
└── Getting-Started.md     # Practical development workflows

Benefits for Developers

This documentation provides developers with:

• Clear Entry Points: Know exactly where to start when fixing bugs or adding features
• Algorithm Understanding: Comprehensive explanations of complex geometric algorithms
• Best Practices: Established patterns for code organization, error handling, and testing
• Performance Guidance: Optimization techniques and complexity analysis
• Integration Patterns: How to properly integrate with Unity's editor systems

The documentation totals over 1,700 lines covering everything from high-level architecture to specific debugging techniques, giving developers the knowledge needed to work effectively with ProBuilder's sophisticated codebase.

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[unity-cla-assistant]

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[codecov-github-com]

Codecov Report

All modified and coverable lines are covered by tests ✅

@@            Coverage Diff             @@
##           master     #617      +/-   ##
==========================================
+ Coverage   33.66%   33.68%   +0.02%     
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  Files         277      277              
  Lines       34785    34785              
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+ Hits        11709    11718       +9     
+ Misses      23076    23067       -9     

Flag	Coverage Δ
mac_trunk	33.47% <ø> (+0.01%)	⬆️
win_trunk	33.36% <ø> (+0.01%)	⬆️

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