Official documentation

We should have some documentation on the official duckdb documentation site: https://duckdb.org/docs/

Start by reviewing the repository description and the official DuckDB documentation site at https://duckdb.org/docs/. The issue does not identify a missing topic, page, file, or acceptance criteria, so first determine what pg_duckdb documentation is needed and where it belongs. Done means the agreed documentation is published on the official site.

Written by the indexing model from the issue text.