Enable show_submodules option in MkDocs config (#379)

Fixes #365

Enables `plugins.mkdocstrings.handlers.python.options.show_submodules`
option in `mkdocs.yml` configuration file for template so that
[submodules of the top-level package are included in autogenerated API
reference documentation by
default.](https://stackoverflow.com/a/74237199)

{{cookiecutter.project_slug}}/mkdocs.yml

@@ -50,6 +50,8 @@ plugins:
           docstring_style: google
           import:
             - "https://docs.python.org/3/objects.inv"
+          options:
+            show_submodules: true
           paths: [src]
   - include-markdown:
       opening_tag: "{!"