Include DuckDB enums and struct on the C API complete reference page.

The duckdb.h contains enums and structs (`duckdb_state`, `duckdb_statement_type`, `duckdb_hugeint`, `duckdb_timestamp`, etc.) that are returned by the C API functions. It would be helpful to have them on the [Complete API](https://duckdb.org/docs/api/c/api) page

Start with the Complete API page linked in the issue and inspect duckdb.h for the enums and structs returned by the C API functions. Compare the existing reference entries with the declarations named in the issue, then confirm that the requested types, including duckdb_state, duckdb_statement_type, duckdb_hugeint, and duckdb_timestamp, appear on the completed page.

Written by the indexing model from the issue text.