Generate apidoc using extension

[sverhoeven]

Now a function in the docs/conf.py is used to generate rst file for the api documentation.
See

python-template/{{cookiecutter.project_slug}}/docs/conf.py

Line 86 in 55129e2

def run_apidoc(_):

There is a Spinhx extension called https://github.com/sphinx-contrib/apidoc which can do this with configuration instead of code.

[fdiblen]

We used sphinx-autoapi extension which is also used+made by ReadTheDocs developers.
done in #287