Refocus README on docs.getdbt.com for setup and configuration

[dataders]

Summary

This trims the README down so it primarily points readers to the maintained dbt documentation for setup and configuration.

It keeps the repository-specific material that still makes sense to live close to the source, especially:

• configuring dbt-duckdb plugins
• using module_paths
• writing custom plugins
• the interactive shell
• the short roadmap

Why

The README had grown into a second, partially duplicated copy of the adapter docs. That makes it easy for the README and the dbt docs site to drift.

The dbt docs updates that this PR is designed to point to are being proposed here:

• Update DuckDB configs docs to match current dbt-duckdb adapter dbt-labs/docs.getdbt.com#8978
• Add DuckDB Fusion adapter docs (v2.0) dbt-labs/docs.getdbt.com#8979

Changes

• replace large setup and configuration walkthrough sections in README.md with links to docs.getdbt.com
• retain repo-specific guidance in the README
• update the retained plugin dependency note for pyiceberg to Python >= 3.10