Use audeer.path() instead of os.path.join() in docstring examples

[hagenw]

As pointed out at #94 (comment) we currently use os.path.join() in the docstrings as audeer.path() is not available as we import it as safe_path() so we can use path as a variable.

But it would be good to use path() in the examples. One workaround might be to inject it with a conftest.py, e.g.

import pytest

import audeer


@pytest.fixture(autouse=True)
def add_pd(doctest_namespace):
    doctest_namespace['path'] = audeer.path

But then we have to make sure to not use the variable path in the docstring examples.

[frankenjoe]

The other option we have is to create some sample files in the fixture as we do in audbackend.