TST/CI: Automatically test documentation when it is built

[Kai-Striega]

Proposed new feature or change:

The CI pipeline should automatically test the code examples in the documentation when it is being built. This will ensure that our documentation remains current.

[MashyBasker]

Hi, I'd like to work on this issue.

To my understanding, we can install doctest and use it on the numpy_financial/_financial.py/financial.py file to test against the examples in the docstrings.

But I cannot understand if I should install doctest in the publish_docs_to_pages.yml or the pythonpackage.yml workflow. Please correct me if I have made any mistake in my understanding.

Thanks.