Add architecture documentation for maintainers (#2633)
Add internal architecture docs covering Cog's core concepts:
- Model source (cog.yaml, Predictor, types, weights)
- Schema (OpenAPI generation and consumers)
- Prediction API (endpoints, envelope format)
- Container runtime (two-process architecture, IPC)
- Build system (config to image pipeline)
- CLI (commands and host-container interaction)
These docs establish shared vocabulary for maintainers and
provide mental models for understanding Cog's design.
Also removes outdated ARCHITECTURE.md which contained
deprecated coglet/monobase content.
