Remove userguide/changelog from API Reference docs

[jabooth]

If we are committing to this as the one source of truth for menpo information outside of the API reference docs (which would be then considered purely a way to explore menpo's fantastic docstrings) we should remove the user guide from there, and make clear that it's just for reading docstrings. We would point heavily to the website from there to make clear there are full user guides available.