docs: add repository README

[manuzhang]

Summary

• Add a root README that explains the workspace crates and the DataFusion integration.
• Show how to query an existing Iceberg table with IcebergTableProviderFactory and point readers to the catalog-backed path for inserts.
• Document the pinned Iceberg dependency and the commands used by CI.

Validation

• git diff --check
• Rust example extracted from the README passes rustfmt +1.98.1 --edition 2024 --check.
• Checked all relative links against the repository.

No Cargo tests were run for this documentation-only change.