DOC: simplify API documentation

[hagenw]

Removes audpsychometric.datasets sub-module, and flatten hierachy in API documentation:

The documentation, that listed the available datasets was moved to the examples in the docstring:

And we also show an example dataset now:

[hagenw]

When building the docs locally, I see that there is a dfiference between the github action and CONTRIBUTING.rst.

I wonder whether there is a reason for that?

python -m sphinx docs/ docs/_build/ -b html -W
python -m sphinx docs/ build/sphinx/html -b html

I changed it to

python -m sphinx docs/ build/html -b html

and

python -m sphinx docs/ build/html -b html -W

to use the same build folder.

The additional -W in the Github action let's the test fails (raise error for warning), which you usually don't want when building locally.