docs: strengthen quickstart and contribution paths

[AetherAI3]

What changed

• Reframed the README around the practical value for quant teams and product engineers.
• Added a real, executable Momentum demo and kept installation, execution, and test commands together in Quick Start.
• Replaced the text-only architecture path with a GitHub-rendered Mermaid flow that makes the host/Nano boundary visible.
• Elevated the trading strategy library as a community on-ramp, with categories, corpus scope, and clear browse/add/propose paths.
• Reworked contribution guidance and added structured issue forms plus a pull-request template.
• Extended wheel verification so CI checks every bundled strategy source and reference IR asset.

Why

The first documentation pass established accurate boundaries. This follow-up makes Nano easier to understand, try, and contribute to without overstating what the reference runtime does.

Validation

• python -m compileall -q nano examples tests
• Executed the Momentum demo with the project import path available; it emits BUY BTC at timestamp=300 (confidence=0.91).
• Ran the README source regression and demo regression directly.
• Checked local Markdown links and git diff --check.

The full pytest suite could not be run in this sandbox because its package index cannot supply pytest; CI continues to run it.