Check documentation examples with doctest or xdoctest

[sverhoeven]

In the docstrings of the code or in Rst files in docs/ directory you can have code examples.

To make sure those examples actually work they can be reformatted into test snippets and run using a Sphinx extension (https://www.sphinx-doc.org/en/master/usage/extensions/doctest.html) or with Pytest plugin (https://github.com/Erotemic/xdoctest#using-the-pytest-interface).

For example in matchms the Sphinx build also runs the doc tests at https://github.com/matchms/matchms/blob/5497b7363af1d555b8d1ab0162e01458ac7fbf53/.github/workflows/CI_build.yml#L129

[fdiblen]

done in #287

[sverhoeven]

Run by doctest in CI or README.dev.md