Expand PineForge Data documentation

[luisleo526]

Summary

• add a PineForge Data documentation hub with architecture, boundaries, guarantees, and task-based navigation
• add a getting-started guide covering installation, exact market resolution, confirmed OHLCV, local backtests, and FastAPI routing
• document the normalized instrument, contract, listing, bar, trade, macro-vintage, request, and low-level engine-stream models
• add a provider usage guide for registry creation, CCXT catalog filtering, historical pagination, live trade sequencing, configuration layers, lifecycle, and errors
• add a backtesting guide for local/remote modes, CLI groups, configuration files, runtime channels, report envelope, and reproducibility
• expand the README with prominent documentation navigation and a contribution routing table, setup, provider rules, and PR checklist
• require documentation updates for public behavior changes in CONTRIBUTING.md
• add a test that checks every relative Markdown link in the README, contribution guide, and docs/

Why

The repository documented individual provider and server contracts but did not provide a coherent user manual for PineForge Data itself. Users had to read source code to understand market discovery, derivative identity, record invariants, live handoff, CLI configuration, report provenance, and the local-versus-server execution boundary.

Validation

• ruff format --check src tests
• ruff check .
• strict mypy src
• pytest (71 passed, 2 Docker-gated)
• relative documentation link validation across all Markdown guides