[duckdb/#18609] - Add the `VARIANT` LogicalType needs documentation

See https://github.com/duckdb/duckdb/issues/18609

Start by reading the linked DuckDB issue 18609 to understand the VARIANT LogicalType documentation request. No documentation file, entry point, or test is named here, so locate the relevant documentation area from that issue. Done means the requested VARIANT LogicalType documentation is added and the site’s documentation checks pass.

Written by the indexing model from the issue text.