Add contributor documentation for pin signature architecture

Created comprehensive internal architecture guide covering:

- Annotation infrastructure (amaranth_annotate, submodule_metadata)
- IOSignature base classes and IOModelOptions
- Concrete pin signatures (UART, GPIO, SPI, etc.)
- simulatable_interface and SoftwareDriverSignature decorators
- Platform consumption patterns (silicon, software)
- Complete flow example from signature definition to code generation
- Guide for adding new signatures and platform backends

This documentation is for contributors who need to understand or extend
the pin signature system, complementing the user-facing guide in
using-pin-signatures.rst.

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

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

Author: robtaylor