feat: Add comprehensive quickstart guide and tutorials

- Add QUICKSTART.md with installation, basic usage, common workflows, and troubleshooting
- Add TUTORIALS.md with 7 step-by-step tutorials covering:
  - Moving Average Crossover Strategy
  - Building Custom Indicator Strategies
  - Backtesting with Performance Metrics
  - Integrating with VectorBT
  - Multi-Timeframe Analysis
  - Creating Custom Indicators
  - Candlestick Pattern Recognition
- Add docs/quickstart.rst for Sphinx documentation
- Update README.md with prominent links to new guides
- Update docs/index.rst to include quickstart in table of contents

Fixes #50

Author: xgboosted