docs: rewrite README for v0.1 — new API, all demos, accurate pipeline

Reflects the current state of the project:
- Single-package install (ignitionai umbrella)
- TrainingEnv class-based API (replaces old callback API)
- Algorithm switching via env.train('dqn'|'ppo'|'qtable')
- 5 demos documented (2D + 3D) with ports
- ONNX bridge explained honestly (Python conversion step)
- Speed control via env.setSpeed()
- Project status, contributing guide, tips

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: salim4n