Add docs for how to document only Python files

[filipwastberg]

I found the work around for how to use the source_dir option correctly in this old issue: #130.

I didn't understand how to use it through the docs so this PR adds an example to the docs.

Not sure if it is in the right place, so feel free to move it. I found quartodoc really helpful and easy to use (once I managed to find the above issue) to document a Python project that didn't end up as a package.

[machow]

Thanks for this PR -- I'm working on some quartodoc issues / features, and will take a look today!

[machow]

LGTM, thanks for taking the time to write this up!

[ecomodeller]

@filipwastberg Nice. I can imagine this type of project structure applies to a wide range of both commercial and academic projects, where the goal is not to create a reusable library, but still being able to make sense of the code. 👍