Extend docstrings of Dataset properties by Examples

[hagenw]

Usually, we add examples to the docstrings of functions/methods. I think it would make sense to also add examples to the cached properties of audbcards.Dataset.

I started with an example in #97.

[ChristianGeng]

I think that writing doctests as in #101 is hightly desirable.
Often I think though that the way in which doctesting is documented is underdeveloped.

CONTRIBUTING states that under the hood doctests are always executed as python -m pytest as this is not selective
on its input directories. pyproject.toml has the --doctest-modules also enabled.

I can run them separately by select the module directory :

pytest --doctest-modulesaudbcards/

But how do you get them more verbose? -vvvv -s have no effectl for me.

In case this is somehow possible then I would find this helpful to have it in CONTRIBUTING.