Simplify API web design

[dustinvtran]

Having a top navigation bar, API sidebar, and tutorials is unnecessary. The biggest change is that I removed all docs on the navigation bar and either made it explicitly part of the API docs (as docstrings) or threw it into the tutorials section.

todo

• change the sidebar and allow more fine-grained pages via a dynamic scrollbar with collapsible sections
• deliberate on how to organize tutorials given API tutorials. should it be "Tutorials" and "API tutorials"? "Tutorials" and "Advanced tutorials"? Maybe shove all API tutorials into api/ sidebar like TensorFlow docs sometimes does?