MkDocs documentation in template omits submodules

[matt-graham]

Describe the Bug

MkDocs documentation in template doesn't include submodules of top-level package in API reference as we don't enable the show_submodules option.

To Reproduce

Create an instance of template, add a submodule to the package with a docstring, generate docs.

Expected Behaviour

Documentation generated for top-level package and all submodules.

Actual Behaviour

Only the definitions in top-level __init__.py file appear in API reference page.

Additional Context

- Cookiecutter version: 
- Operating system:
- Python version: