Add user guide and API documentation

[astrojuanlu]

Most of the functions have proper docstrings but still I have to build the documentation, maybe using Sphinx and putting it in a separate gh-pages branch.

• Generate API documentation
• Write short user guide with links to examples

[jorgepiloto]

After #29 I was expecting this web https://aeropython.github.io/scikit-aero/ to show the sphinx_rdt_theme, however it is still using the old solar one