docs: clarify period-based vs trade-based metrics

[ranaroussi]

Summary

• Added documentation explaining that metrics like win_rate, consecutive_wins, payoff_ratio, and profit_factor are period-based (not trade-based)
• Updated Python version badge from 3.6+ to 3.10+
• Updated requirements section to match current pyproject.toml

Context

Addresses concerns raised in #493 where a user noted that these metrics could be misleading for discretionary traders expecting trade-level statistics.

The documentation now clarifies:

• These metrics analyze return periods, not discrete trades
• This is valid and useful for systematic strategies
• It's consistent with how all return-based analytics (Sharpe, Sortino, etc.) work

Changes

1. Added "Important: Period-Based vs Trade-Based Metrics" section
2. Updated version badges and requirements to reflect v0.0.78 dependencies