Add information on how to measure code coverage to the guide

[darrenburns]

Create a separate page in the guide section of the documentation which explains how to configure code coverage (i.e. via coverage run -m ward).

[ThatXliner]

Is there a pytest-cov-like plugin for Ward? Or do we have to use coverage.py?

[darrenburns]

@ThatXliner you'll have to use coverage.py. You can see an example in the GitHub Actions folder in this repo