Move CLI from `__main__.py` to `entrypoint.py`

[DunklesArchipel]

Checklist

• [x] Added an entry in doc/news/.
• [x] Added a test for this change. (not required)
• [x] Updated the documentation. (not required)

Changes the entry point such that the doctests of the CLI are no longer ignored during testing.

Fixes #161

Dependencies:

• [x] #169
• [x] #175

[DunklesArchipel]

not sure is __main__.py and entrypoint.py should share the same doctest at the beginning of the document.

[saraedum]

@DunklesArchipel please merge if you agree with the changes here.