ci: check that `Series.str` methods all appear in the api reference

In [utils/check_api_reference.py](https://github.com/narwhals-dev/narwhals/blob/main/utils/check_api_reference.py), we check that all methods are included in the API reference

It looks like we don't do this for namespaces though - could we amend the `check_api_reference.py` file to check that all the `Series.str` methods are also documented?