Execute docs examples in CI

[MartinThoma]

I want the docs to be complete, meaning all code blocks have all imports.

See https://pypi.org/project/pytest-examples/ - I think it's used by pydantic

That should also ensure that we don't have deprecated code in there + that no typos happen.

[stefan6419846]

Sphinx seems to have some built-in support for something like this as well: https://www.sphinx-doc.org/en/master/usage/extensions/doctest.html