Autogenerated API documentation

[galenseilis]

• Ciw's online documentation is already quite good, but it doesn't have the same autogenerated API documentation that I think users have come to expect from many other packages.
• Getting Sphinx to generate API docs will require plugins, such as naploleon.
• Will need to expand on the Numpy-compliant docstrings in the codebase.
• Should add type annotations in most places as well.