Split liquid market sources into asset-class references #68

Replace the combined source tables in docs/providers/market_data.md with standalone reference pages for equities, ETFs, futures, options, foreign exchange, and cryptocurrency. Keep market_data.md as a concise landing page so existing links remain valid. Give every page the comparison dimensions defined in the specification and map wrapped entries to exact provider classes from the registry. Update navigation and the provider index.

Audit candidates from the 2023 reference against current official sources. Retain only reproducible APIs, bulk downloads, exchange products, and institutional delivery products. Keep promising missing-adapter recommendations private.

Verification:

• uv run mkdocs build --strict
• uv run pytest tests/test_provider_registry_contract.py -q
• Every wrapped row names an exported provider class; comparison-only rows say No.
• Every retained external link reaches the intended official product or has recorded automated-access evidence.

Done means the six pages and landing page are navigable, consistent, and contain no unverified or duplicate source claims.

Plan reference: ml4t-data-dev/.workspace/work/asset-class-source-reference/plan.md - Issue 1 in milestone 0.2.1 - Asset-class data-source reference.