Add architecture and simulation documentation

Created two major new documentation guides:

1. **architecture.rst** - Comprehensive architecture overview covering:
   - High-level design flow with ASCII diagrams
   - Core components (signatures, annotations, packages, platforms, steps)
   - Detailed flow from Python → RTLIL → target outputs
   - Pin allocation flow diagram
   - Annotation system architecture
   - Package system architecture
   - Configuration system
   - Extension points (custom signatures, steps, packages, platforms)

2. **simulation-guide.rst** - Complete simulation usage guide covering:
   - Basic simulation workflow
   - What happens during simulation (6-stage process)
   - SimPlatform internals and automatic model matching
   - Port instantiation and clock/reset handling
   - Generated main.cc structure
   - Configuration options
   - Simulation commands (build/run/check)
   - RTL Debugger integration
   - Event logging for automated testing
   - Customizing simulation (adding models, custom drivers)
   - Performance tips and troubleshooting
   - Complete working example

These fill major documentation gaps identified in the coverage analysis,
providing both architectural understanding and practical simulation guidance.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: robtaylor