[doc-only] Add developer guide for cuda.core

[Andy-Jost]

Summary

This PR adds a developer guide for Python and Cython code in cuda/core. The guide establishes conventions and best practices to maintain code quality and consistency across the codebase.

What's Included

The guide covers 10 major areas:

1. File Structure - Ordering of elements (SPDX headers, imports, __all__, classes, functions)
2. Package Layout - Organization of .pyx, .pxd, and .py files
3. Import Statements - Five-group organization with alphabetical sorting
4. Class and Function Definitions - Ordering within classes (dunder methods, methods, properties)
5. Naming Conventions - Project-specific patterns (Cython c_ prefix, type suffixes)
6. Type Annotations and Declarations - Modern PEP 604 union syntax, Cython declarations
7. Docstrings - NumPy docstring style with Sphinx cross-references
8. Errors and Warnings - Custom CUDA exceptions, error handling patterns
9. CUDA-Specific Patterns - GIL management for CUDA driver API calls
10. Development Lifecycle - Two-phase development (Python first, then Cython optimization)

Key Principles

• PEP 8 and PEP 257 Base: References these as normative standards, documenting only project-specific extensions
• Guidelines, Not Rules: Emphasizes readability and maintainability over rigid enforcement
• Codebase-Aligned: Examples reflect actual patterns from the codebase

File Added

• cuda_core/docs/source/developer-guide.rst