docs: Add docs for DuckDB-only functions introduced in 1.0

[JelteF]

Description

We added bunch of DuckDB-only functions in this release. Like time_bucket, strftime, strptime. We should add documentation for those so people know they exist. strftime and strptime should probably also have a note in the types.md file, under "limitations", because these functions primarily exist as a workaround for to_char etc not existing.

Related to this comment where a user asked about to_char: https://github.com/duckdb/pg_duckdb/issues/615#issuecomment-2817506124