Add architecture documentation #56

There is no architecture documentation. Contributors have to read the 800 line log_setup.py to learn how records flow, where context is attached and how the optional pieces fit.

Add docs/architecture.md, linked from the README, covering:

1. Module layout and public API.
2. How a record flows from a logging call to each output, including the queue path.
3. How context is layered (global context, adapter, extra) and why it uses contextvars.
4. The design decisions and constraints that are not obvious from the code: filters per handler, optional dependencies, python-json-logger version differences, colour handling and the formats.
5. How to extend the package, and the testing and release approach.

Use Mermaid diagrams.

Generated by Claude Code