Break up the usage documentation into separate pages: basic usage
(Training your Pooch with some elements removed), single file downloads,
downloaders, processors, and advanced tricks. Moved the processor and
downloader specifications from the docstring of Pooch.fetch to the
respective pages and link to them from the docstrings. Also did general
updates to the docstrings (for example, to include the new retrieve
function) and tutorials. Separated the side menu into Getting Started,
User Guide, and Reference Documentation like in all other projects.

Add an intermediate level tutorial

Move things between them so that the beginner tutorial is really minimal

Start with retrieve and include links to setting up Pooch through the
beginner tutorial

Added some docs explaining what the return of create is.