Split README into user guide + reference docs

[darthfork]

The README is fine for a prototype, but an official ruleset should separate quickstart, API reference, toolchains, and design notes.

Tasks:

• Keep README focused on overview and quick start.
• Add docs/ for installation, rules reference, toolchains, platforms, and troubleshooting.
• Document module-extension usage thoroughly.

Acceptance criteria:

• Users can find install, API, and troubleshooting docs without reading source.

Parent issue: #5